Summary

Clinical Pharmacokinetics 3: 294-312 (1978) © ADIS Press (1978)

Monitoring Serum Theophylline levels

Leslie Hendeles, Miles Weinberger and George Johnson

College of Pharmacy. Pediatric Allergy and Pulmonary Division. and Clinical Pharmacology and Toxicology laboratory. University of Iowa. Iowa City

Recent definition of the pharmacodynamics and pharmacokinetics of theophylline have in­creased the safety and efficacy of the drug for acute bronchodilator therapy and have greatly expanded its potential as a prophylactic agent in the management of chronic asthmatic symp­toms. Specifically. benefit and risk from theophylline have been demonstrated to relate directly to serum theophylline concentration. which is itself a function 0/ not only the dose but also the elimination characteristics 0/ the drug in the individual patient.

When used to treat acute symptoms. an initial loading dose of theophylline based on a volume of distribution 0/0.5L/ kF? (range - 0.3 to O. 7L/ kg) is required to rapidly attain maxi­mum bronchodilator effect. There are wide interpatient differences in elimination rate. Dosage for continued therapy must be matched to the rate 0/ elimination in the individual patient. which can be expressed as clearance. Under normal circumstances. this can only be done em­pirically by monitoring serum theophylline concentration at intervals and adjusting dosage un­til a steady state is reached with the serum theophylline concentration within the 10 to 20)Jg / ml therapeutic range.

During long term therapy. product /ormulation must be carefully considered; sustained release preparations. if completely and reliably absorbed. offer therapeutic advantage. par­ticularly for children. The most acceptable way to determine final dosage for long term therapy is to begin with doses sufficiently low to allow virtually universal acceptance 0/ the medication. although optimum benefit will be obtained in very few. Gradual increases in dose at 3 day in­tervals until average doses are reached. if tolerated. minimises the frequency with which serum theophylline concentrations need to be measured. These doses should then. however. not be maintained or increased further without measurement 0/ serum theophylline concentration. Final dosage adjustment can then be made. Serum theophylline measurement is there/ore es­sential for optimum management of chronic asthma and. when rapidly available. increases the utility of theophylline for acute therapy.

Six different basic methods for measuring theophyfline in serum or plasma have been developed and multiple modifications 0/ many 0/ these have been utilised in various laborato­ries. 0/ greaiest relevance are: (J) modifications of the traditional extraction methodology and measurement of ultraviolet absorbance first reported by Schack and Waxler in 1949; (2) high pressure liquid chromalOgraphy uf which the reverse phase technology has become the most popular because of its commercial availability; and (3) the enzyme immunoassay which has re­cently been released and appears to have distinct advantage/or the average c!inicallaboratory with regard to cost. specificity. ease of operation. speed of the assay and potential application 0/ the equipment/or assaying drugs other than theophylline.

Monitoring Serum Theophylline Levels

Measurement of the concentration of a drug in body fluids has become useful for certain drugs that have in common a narrow therapeutic index and variable rates of elimination, thus necessitating in­dividualisation of dosage requirements. Drugs in this category include the cardiac glycosides, antiar­rhythmic and anticonvulsant drugs, aminoglycoside antibiotics and lithium. Theophylline, a drug used without routine monitoring of serum levels for most of its 40 years of popular usage, has recently been added to this list. The availability of over 200 theophylline-containing preparations has in the USA added both to confusion and a certain degree of casualness in the use of this potent and potentially toxic agent. Recent data, however, have demonstrated that theophylline fulfJIls the criteria for therapy likely to benefit from routine monitoring of serum drug concentrations. Specifically, efficacy and toxicity both relate to serum theophylline concentration; the thera­peutic index of theophylline is low; and dosage re­quirements are highly variable because of interpatient differences in elimination rates.

1. Rationale for Monitoring Serum Theophylline Concentrations

I . I Relationship Between Clinical Response and Serum Theophylline Concentration

The bronchodilator effect of theophylline in hospitalised patients with reversible airways obstruc­tion has been shown to increase with serum con­centrations over a range of 5 to 20pg/ml (Mitenko and Ogilvie, 1973; Levy and Koysooko, 1975).

The relationship between serum theophylline con­centration and response has also been demonstrated in exercise induced bronchospasm, a nearly universal manifestation of asthma (Pollock et ai., 1977). Serum concentrations above I Opg/ ml inhibit bronchospasm in response to standardised treadmill exercise stress, with an even more profound inhibitory effect at serum concentrations above 15pg/ml (fig. 1). The

40

20

*- 0 o

-> .~ -20 Cl c: '" 13 -40 ill .~

~ -60

~ -80LL __ -L __ ~ __ ~ __ ~ __ ~ ____ __

o 5 10 15 20 25

Serum theophylline (pg/ml)

295

Fig. 1. Relationship between serum theophylline con­centration and exercise-induced bronchospasm following standardised treadmill exercise performed before and at 2, 4 and 6 hours following 7.5mg/kg of theophylline administered to 12 children.

The shaded area represents mean values for the intervals of serum theophylline concentration. The dotted line indicates a conventionally accepted decrease in pulmonary function regarded as clinically important. The V 50 is the flow rate at 50% of vital capacity during a maximal forced expiration. The mean Spearman rank correlation coefficient was 0.71 (p < 0.001)[Poliocketal., 1977).

effect of theophylline in preventing exercise-induced bronchospasm is maintained with continuous dosing and development of tolerance does not appear to oc­cur (Bierman et ai., 1977).

Prevention of the symptoms of chronic asthma has also been demonstrated when serum theophylline concentrations are maintained on an around-the-clock basis at levels within the 10 to 20pg/ ml range (Wein­berger and Bronsky, 1974, 1975; Hambleton et ai., 1977; Jenne et ai., 1972). In a double-blind controlled evaluation, theophylline alone in individualised doses resulting in serum theophylline concentrations over I Opg/ ml, was associated with much better control of the disease among 12 children, compared with either

.. 1It10f]itoring Serum. ThElo~h."lIine .Leyels.

60

50

'" Q) 30 u c: ~ " 20 u u 0 -0

m 10 .c E " z 0

p > 0.05 : p < 0.05

I

I ~ Adrenaline inj.

28 Substitute drugs

.296.

Placebo Peak serum theophylline (averaging 6.5"g/mi)

Peak serum theophylline (averaging 13"g/mi)

Fig. 2. Frequency and severity 'of asthmatic symptoms during 1 week's treatment with each of placebo, an ephedrine·

theophylline combination in conventional doses that resulted in serum theophylline concentrations averaging 6.5"g/ml, and in· dividualised theophylline doses that resulted in serum theophylline concentrations averaging 13"g/ml.

Asthmatic symptoms -during each 1·week period were promptly treated when necessary with inhaled isoprenaline (isoproterenol); if symptoms were not promptly relieved, adrenaline (epinephrine) was administered subcutaneously. If the patient was unresponsive to these measures, known medications were substituted for the double·blind medications (Weinberger and Bronsky. 1975). --------

placebo or a conventional dose of theophylline or Deaths have been most frequently reported among .. J:heophxlline.plus .ephedrin(!. (fig. 2L _ ..... __ ._"., .. .small children .who havueceived.multiple, adult-size .. _ .. _

1.2 Relationship Between Toxicity and Serum Theophylline Concentration

Theophylline has the potential for. a wide range of adverse effects. At one end of the spectrum are minor caffeine-like side effects that appear to have little direct relationship to serum concentration and are. associated with rapidly acquired tolerance during long term therapy. Adverse effects associated with serum concentrations above' 20llg/ ml do not appear to be associated with the development of tolerance and in­clude nausea, headache, diarrhoea and, at higher levels, vomiting, gastrointestinal bleeding, seizures and cardiac arrhythmias (Hendeles et aI., I 977a; Zwillich et aI., 1975; Jacobs et aI., 1976).

doses, commonly administered as suppositories. Ir­ritability, vomiting of coffee-ground material and seizures, from which the patient does not regain con­sciousness, have characterised the common clinical course in such cases. Recently, similar deaths in adults have been reported from a university hospital with a large and highly competent pulmonary disease service (fig. 3)[Zwillichetal., 1975]. Of the 8 deaths that occurred over a I 0 month period, seizures pre­ceded death in all cases and in only one were other ad­verse effects of theophylline apparent in the medical record prior to the seizure. Patients with seizures generally had considerably higher serum theophylline concentrations (mean 541lg/mO than patients having less severe adverse effects (mean 35Ilg/mO. Patients having no adverse effects averaged serum concentra­tions under 20llg/ml. In another report, 75 % of

Monitoring Serum Theophylline Levels

patients with serum concentrations over 2Spg/ml ex­perienced adverse effects that were uncommon be­tween IS and 2Spg/ml and not observed below ISpg/mi (Jacobs et aI., 1976).

1.3 Variability of Dosage Requirements

The relationship between elimination rate and dosage requirement was first demonstrated by Jenne et al. (1972) for adults. A similar wide range in dosage has subsequently been confirmed for children

70 •

60 • -- Median

-'- • - Died

50 , 40 ..

• 30 • •

• • ••

20 ~ , •

10

•

o'----ro----.-----.--Seizure Other No

adverse adverse effects effects

Fig. 3. RelationShip between serum theophylline con­centration and seizures.

Eight patients with seizures were identified on the pulmon­ary service at the University of Colorado Hospitals over a 10 month period; 4 of them died, having never awakened from seizures. Doses were 'conventional'. Among other patients whose serum theophylline concentration was being monitored, other adverse effects including nausea, vomiting, and headache occurring without seizures at a significantly lower mean serum concentration, and the patients without identifi­able adverse effects had a mean serum concentration that was less than 20jlg/ml; a significantly lower value (Zwillich et al .. 1975).

297

Table I. Continuous theophylline dosage for acutely ill

patients following an initial loading dose

Patient age/clinical condition

Children under 9 years of age

Children over 9 and otherwise healthy adults who smoke

Otherwise healthy non-smoking adults

Cardiac decompensation including cor pulmonale

Infusion rate (mg/kg/h*)

0.85

0.75

0.5

0.3

liver dysfunction 0.2

liver dysfunction and cardiac decompen- 0.1 sation.

* These are guidelines for initial infusion rates of theophylline (aminophylline = theophylline/0.85). Final dosage requirements may be higher or lower and should be guided by serum theophylline measurement.

(fig. 4) [Ginchansky and Weinberger, 1977]. Patients with slower plasma clearance of the drug require less frequent dosage during long term therapy than those with more rapid elimination. This appears to be due primarily to variation in the rate of hepatic biotransformation. While rates for elimination re­main relatively constant over time under normal cir­cumstances (fig. S), alterations in hepatic function may serve as a confounding factor for theophylline clearance and thereby theophylline dosage require­ments.

Patients with hepatic cirrhosis have, on the average, much lower theophylline plasma clearance than do individuals with normal liver function (Piafsky et aI., 1 977a). The elimination half-life of theophylline is prolonged in patients with acute pulmonary oedema (Piafsky et aI., 1977b) and in severe congestive heart failure (Jenne et aI., 1975). Theophylline toxicity as a result of excessive serum levels has often been associated with the use of recommended dosage regimens in patients with liver or cardiac decompensation, including cor pulmonale

Monitoring Serum Theophylline Levels

r sL E ----Cl 5 ::1._ - Cl -g~ > Cl

.!l! E ~-

41-U'O

'" '" c: ~ o~

'';:::; (/) 3f-

CO'c E'E

2f-"''0 u'" c: '" o Vl U 0 '0'0 1 f-0'" .-~ --"'0 0::_

• • .......... Peaks (r = - 0.80)

(}--o Troughs (r = - 0.85)

298

1

I 0

.2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Clearance (ml/kg/min)

Fig. 4. Theophylline clearances among 21 children with relation to the ratio of peak and trough serum concentrations achieved to the 6-hourly dose being administered on a continuous around-the-clock basis.

A significant relationship was observed between the serum concentration/dose ratio and clearance accounting for the wide range of doses required to maintain serum concentrations within the therapeutic range (Cummins et aI., 1976).

(Hendeles et aI., 1 977a; Weinberger et aI., 1976; Zwillich et aI., 1975). In patients with these concur­rent functional abnormalities, theophylline dosage regimens must be markedly reduced in order to pre­vent-toxicity (see table I). Fever has been shown to . decrease the clearance of antipyrine (Elin et aI., 1975) and appears to have been associated with transient slowing of theophylline elimination in 2 cases (Ginchansky and Weinberger, 1 977; Matthay et aI., 1976).

Troleandomycin (triacetyloleandomycin), a macrolide antibiotic, decreases theophylline clearance in patients with normal liver function by about 50 % (fig. 6) [Weinberger et aI., 1977] and a related macrolide antibiotic, erythromycin, has been sug­gested to have a similar though less marked effect (Kozak et aI., 1977; Pfeifer et aI., 1978). The ac­cumulation to toxic theophylline levels appears to oc­cur in the first 36 to 48 hours of therapy. A decrease in theophylline clearance can be measured 24 hours after standard doses of erythromycin are begun (Pfeifer et aI., 1978).

3

First clearance (ml/kg/min)

Fig. 5. Relationship of clearances among 16 children with chronic asthma performed at 2 different points in time (mean interval = 5 months).

The dotted line represents identity. Clinically important variability of clearance over time was not observed in these patients (Ginchansky and Weinberger, 1977).

Monitoring Serum Theophylline Levels

The serum concentration attained from an initial or loading dose of a drug that has rapid absorption, such as theophylline, is related more to the apparent volume of distribution (V d) of the drug (the apparent space into which the drug diffuses), than to its clearance from the body. The volume of distribution of theophylline appears to have considerably less variability (0.3 to O.7L/kg) than the rate of elimina­tion and averages about 0.5L/kg. It is similar for otherwise healthy volunteer adult asthmatics (Hen­deles et aI., 1978a) and patients with hepatic cirrhosis (Piafsky et aI., 1 977a) and for children (Ellis et aI., 1976). Since the concentration of a drug (C) attained following rapid absorption, is equal to the dose ad-

40

E ---2 30 c o . ., ~ E Q) u c o ~ 20 ~ >-~ c. o Q)

-S E ~ 10

'" 2l j!!

'" >-"0

'" Q)

299

ministered (D) divided by the apparent volume of dis­tribution (V d)

C D

Vd (Eq. I)

each 1 mg/kg of theophylline administered in a rapidly absorbed form (e.g. intravenous, oral solu­tion, or rapid dissolution tablet) will result in an average 2J.1g/ml increase in the serum theophylline concentration. For continued dosage of theophylline, however, much more variability is observed because of the wide range in aged-related and individual plasma clearances. Thus, among 157 children and 33

E.V.

Troleandomycin (250mg qid) Control

~ O~--~----~---r----~---r----~--~ o 10 20 30 40 50 60 70

Rate of theophylline infusion (mg/h)

Fig. 6. Steady state serum theophylline concentration resulting from continuous intravenous infusion of theophylline at 2 dif­

ferent doses to 6 patients while receiving troleandomycin, 250mg qid, and during the control period when no troleandomycin or related antibiotic was being taken.

A 50% reduction in clearance is indicated by the relationship of serum theophylline concentration to dosage; as a result,

steady state serum concentrations were generally twice as high during the period when troleandomycin was being administered as compared with the control period (Weinberger et aI., 1977).

· .. 300

:;: .. J n=8' n=36 n=33 n=37 n=42

<1l v

" "-OJ ~

28 "-OJ -E' ~ r---...... a> 20 ....... .-. -CJ) 0

,........

~ " L-

a> .S 12 '5. -" ,0.

2 4 ~

0.2-2 2-6 6-9 9-12 12-16 Adults

Age(y)

Fig. 7. Mean and 2 standard deviations for dosage requirements to maintain serum theophylline concentrations within the 10 to 20pg/ml therapeutic range,

Mean doses decline beyond the age of 9 years; with the adolescents and preadolescents differing significantly from adults and younger children, although considerable variability in dosage requirements persists within all age groups (Wyatt et aI., 1978).

adults in whom serum concentrations were used to guide. long term oral therapy, doses averaging 24mg/kg/day were observed for children under 9 years with a progressive decrease in weight adjusted doses through adolescence until dosage requirements

2. Use of Seru m Theophy [line Concen Ira lions to Guide Intravenous or Emergency Therapy

2. 1 Loading Dose

of I3Ing/kg/ day were observed for adults (fig. 7)' As previously stated in section 1.3 each I mg/kg [Wyatt et ai., 1978]. The actual dosage requirements administered in a' manner that rapidly enters the

. for.adults has.been.observed.to range, fromJess .than_ .systemic. circulation. results in a 2)lg/ mLincreasein .. - .. -400mg/ day to as much as 2g/ day. serum concentration. Thus, 7.Smg/kg administered

The use of fixed dose recommendations is by a 30 minute intravenous infusion, by an oral solu­therefore made difficult since an average dose, even tion or an uncoated tablet with rapid dissolution when adjusted for age, will result in as much as 20 % characteristics, will result in a peak serum theo­of that population being at risk for toxicity (fig. 8) phylline concentration of approximately IS)lg/ml [Wyatt et ai., 19781. If the average dosage for children (fig. 9)[Hendeles et ai., 1977c1. The administration of under 9 years is applied to children of all ages, a theophylline solution rectally would result in simi­progressively increasing risks for toxicity will be ob- lar time concentration relationships (Lillehei, \968). served among children aged 9 to 16 (fig. 8). More rapid intravenous administration will result

The above findings strongly support the use of in a transiently higher serum concentration because serum theophylline concentrations to monitor ther- of the finite time required for distribution from a apy with this drug. The relationship between serum central compartment into the whole body theo­concentrations and effect establishes a narrow thera- phylline distribution space described by the apparent peutic index. Moreover, since the rate of elimination volume of distribution of O.SL/kg. On the other and therefore dosage requirements vary over a wide hand, delayed gastric emptying, as when oral medica­range, individualisation of therapy offers the op- tion is taken with food in the stomach, might result in timum chance for benefit with minimal risk. a delayed and somewhat lower peak serum concentra-

Monitoring Serum Theophylline Levels

100

80

~ 60

'" . ., ., c. '0 40 E ~

c'f. 20

Theophylline dose (mg/kg/day)

301

8r-----------------------------------------------------------------~

E " Cl 2: Q; > ~

'" .~ '5. .J::. c. 0

'" -5 E :J Ii;

CJ)

20

2

'. Mean Doses:

.......... IV 7.5mg/kg (n=12)

().-_oO Gyrocap 8.0 mg/kg (n=12)

•.....• Tablet 7.5mg/kg (n=6)

.... _ .... Elixir 7.4mg/kg (n=6)

Time (h) 9~ ________________________________________________________________ ~

Fig. 8. Cumulative frequency distribution for serum theophylline dosage likely to exceed serum theophylline concentration (STC) of 1Ollg/ml among patients less than 9 years of age, for doses likely to result in serum theophylline concentrations greater than 20llg/ml among the s~me patients, and for doses likely to result in serum theophylline concentrations in excess of 20llg/ml among adolescents and preadolescents. For adults, the curve would have shifted even further to the left (Wyatt et al., 1978).

Fig. 9. Serum theophylline concentration over time following administration of intravenous theophylline, a solution (elixir! of theophylline, an uncoated tablet, and a sustained-release preparation (Slo-Phyllin Gyrocaps) - all administered in similar dosage.

Completeness of absorption was similar for the 3 non-parenteral products, as indicated by similar areas under the serum con­centration-time curves. The rate of absorption for the tablet and solution were similar and closely paralleled the intravenous dose which was administered by constant infusion pump over a 30 minute period (Hendeles et aI., 1977c).

Monitoring Serum Theophylline Levels

tion as a result of slower (though equally complete) absorption (Welling et a!., 1975). Enteric coated theophylline products may result in even more pro­found delay in absorption, which may be incomplete (Waxler and Schack, 1950). Even under ideal cir­cumstances, however, variability in peak serum theophylline concentrations resulting from a loading dose will vary to some degree as a function of the variability of the volume of distribution which has been reported to range from 0.3 to 0.7L1kg (Jenne et a!., 1972; Ellis et a!., 1976). For this reason, the load­ing dose should aim for a mean serum concentration near the lower end of the therapeutic range; e.g. 10 to 15}lg/ mi.

These data would result in a recommendation for a loading dose of 5 to 7.5mg/kg if the initial serum theophylline concentration was at or near O. Unfor­tunately, this is not a common occurrence and the usual history under emergency conditions does not readily provide data from which to predict the initial serum concentration (fig. I 0) [Weinberger et a!., 1976]. Ideally, then, the initial serum concentration should be rapidly determined prior to administration of a loading dose. The desired loading dose can then be determined as follows:

DSC- MSC* (Eq. 2) Loading dose

This returns us to the previous statement that (using the mean volume of distribution of 0.5L1kg) each mg/kg administered as a loading dose will in­crease the serum concentration from the initial value by about 2}lg/ml. Choosing a conservative value for the desired serum theophylline concentration of 12}lg/ml, allows for a lower than average volume of distribution. Using ideal body weight for obese patients also avoids attaining excessive serum con-centrations from this calculation.

302

Waiting at least 45 minutes following complete absorption of the loading dose permits adequate time for distribution and a repeat measurement of serum theophylline concentration can then detect those patients for whom an additional loading dose can be safely administered (as may be required in patients in whom theophylline distributes to a larger extent).

When measurement of serum theophylline con­centrations is not immediately available, patients who have already received theophylline may be given small loading doses (e.g. 2.5mg/kg) with consider­able caution.

2.2 Continued Therapy

The optimym method to maintain serum theophylline concentrations once a therapeutic level is achieved with a loading dose, is by a continuous infu­sion at a rate that parallels the elimination rate at the desired serum concentration. Unfortunately, the variable plasma clearances among individuals result in variable dosage requirements for continued ther­apy. A much publicised, though invalid recommenda~ tion, was based on the data of Mitenko and Ogilvie (1973) which recommended approximately 0.75mg/· kg of theophylline (expressed in their article as 0.9mg/kg/h of aminophylline) as a constant infusion to maintain serum concentrations at an average lO}lg/ml.

Subsequent data has shown, however, that this in­fusion rate is more likely to result in an average serum concentration of 20}lg/ml among hospitalised adults and many patients will attain steady state serum concentrations sufficiently high to place them at risk for serious adverse effects, including seizures (fig. I J) [Hendeles et a!., 1977a; Weinberger et a!., 1976]. Therefore, initial steady state infusions should be based on the age and clinical condition of the patient (table I). Adults require, on the average, lower infusion rates than children and the presence of heart failure, liver disease or chronic obstructive pulmon-

• DSC = desired serum concentration; MSC = measured ary disease with cor pulmonale may decrease serum concentration. theophylline clearance resulting in higher serum con-

Monitoring Serum Theophylline Levels 303

Theophylline dose during prior 24 hours from initial history (mg/kg) 10r-________________________________________________________________ ~

50 @W Therapeutic range ~

p < 0.001

40

30

10 : :

.::

0 No toxicity Mild Potentially Severe

serious (14.6 ± 4 (27.6 ± 4.2 (40.5 ± B.6 (46.5 ± 5.6 N ~ 32) N ~ 6) N ~ 6) N ~ 6)

11~ ________________________________________________________________ ~

Fig. 10. Relationship between the measured serum theophylline concentration and the history of theophylline dosage among 19 patients seen at the University of Colorado Medical Center's emergency room. There was little correlation of history of prior theophylline intake and resultant serum concentrations (Weinberger et al .. 1976).

Fig. 11. Serum theophylline concentrations resulting from intravenous theophylline dosage to adults in an intensive care unit. Average doses used were O.7mg/kg/h of theophylline (O.9mg/kg/h of aminophylline). Using these previously recommended

infusion rates. 17 of 49 patients experienced varying degrees of theophylline toxicity as a result of elevated serum concentrations.

The relationship between the degree of toxicity and serum theophylline concentration was supported by these data (Hendeles et ai., 1977a).

centrations than usual (see section 1.3). In fact, the variability in plasma clearance among patients is so great that no constant infusion can be recommended that can reasonably predict both optimum therapeutic efficacy and safety. For adults in particular (children

elesser frequency of heart failure, cor pulmonale and liver disease), serum concentrations must be monitored if theophylline is to b~ continued at full therapeutic doses for more than 12 to 24 hours.

Ideally, the initial infusion rate should be started and serum concentrations at the beginning of the in­fusion and 4 to 8 hours later compared to determine the direction taken by the serum concentration. Em­pirical adjustments of the infusion rate followed by repeat serum measurements can then maintain serum concentrations within the therapeutic range. This dose qm subsequently be administered orally using an appropriate interval for the preparation used (see sec­tion 3. J).

The goal of emergency or acute therapy is to pro­vide one or more loading doses to rapidly attain thera­peutic serum concentrations, while minimising the risk of serum concentrations exceeding 20)lgl m1. Beginning with an appropriate continuous infusion based on mean pharmacokinetic data, serum theo-

"phylline- concentnl.tions are __ then~monitored. suffi __ ciently to adjust the infusion to that which results in a steady state serum concentration within the thera­peutic range (fig. 12). This is easily done when. serum theophylline concentrations are rapidly available; as is feasible with many of the current analytical methods described below.

3. Use of Serum Theophylline Concentrations . to Guide Continued Oral Medication

3.1 Theophylline Preparations

Theophylline is inherently well absorbed (Hen, deles et a1., I 977b). The absorption of plain uncoated tablets is almost as rapid as a solution and virtually 100% of the drug is absorbed in these forms (fIg. 9).

304

Peak serum theophylline concentrations following a single oral dose on an empty stomach OCCurs about 2 hours following the dose. Absorption may be some­what slower, but no less complete, when food is pre­sent (Welling et a1., 1975). The presence of strong bases and alcohol do not increase the rate and/ or completeness of absorption, contrary to commonly held claims (Weinberger and Riegelman, 1974; Koysooko et aI., 1975). Thus, so-called 'salts' of theophylline such as aminophylline, oxtriphylline (choline theophyllinate), theophylline calcium salicyl­ate, etc., offer no therapeutic advantage over modern theophylline preparations but do serve to create con­fusion when dosage is identified in terms of the 'phan­tom' drug; e.g. I OOmg of aminophylline may be from 78 to 86mg of theophylline and 100mg of ox­triphylline contains approximately 64mg of theo­phylline. Ideally, these misllabeled products should not be used in order to avoid medication errors. Un­fortunately, properly labelled intravenous and rectal solutions are not available.

Although solutions or solid dosage forms with rapid dissolution characteristics appear to undergo rapid and complete 'absorption, coated tablets or ·pro--- --­ducts designed for sustained release will have their own individual absorption characteristics which should_ Q(,U4tm.tiH.e.<!J:>.eJore ~1i!lic(l.!_ll~~(Hendeles et aI., I 977c). For example, enteric coated amino-- -----phylline tablets have previously been demonstrated to be erratically absorbed and their continued clinical use is not recommended. Oxtriphylline (CholedyI) is identified as partially enteric coated and is charac­terised by a 1. to 2 hour delay prior to the onset of ab­sorption with this lag period being longer for the 200mg than the 100mg tablets (Hendeles et al., I 977c). Once absorption begins, however, it progresses rapidly and thus these products cannot be interpreted as sustained release_

Sustained-release preparations, when reliably and completely absorbed, offer a potential advantage by allowing longer dosing intervals with less variation over a dosing interval during continued therapy. Of the various sustained-release preparations currently available in the USA, three (Slo-Phyllin Gyrocaps,

Monitoring Serum Theophylline levels

Theophyl S-R and Theo-Dur) have been shown to be completely and reliably absorbed (Hendeles et aI., I 977b).

Dyphylline (dihydroxypropyltheophylline; gly­phylline) is a xanthine derivative which is sometimes classed as a theophylline product. This drug, how­ever, is stable and does not produce theophylline in vitro or in vivo. Its pharmacodynamic and pharma­cokinetic characteristics are not as well known as theophylline, although preliminary data have revealed a very rapid rate of elimination, making it inappropri­ate for continued therapy. Serum dyphylline levels can be measured, but data are unavailable to identify a therapeutic range (Hendeles and Weinberger, 1977).

OlE c::'-'5.~ -as 2'~

..c::1!! E~ ",0 ~ c:: Ol 0 cnoor,---.,.---.,.-

305

3.2 Adjustment of Dosage

Initiation of long term oral theophylline therapy may be associated in some patients with mild tran­sient caffeine-like side effects which are unrelated to serum concentration. The initial dose should be small (the lesser of 400mg/day or 16mg/kg/day) and slowly increased at intervals no shorter than every 3 days (Hendeles et aI., 1978b). When this dosage schedule is followed, only about I % of children and less than 5 % of adults will not tolerate doses that maintain serum theophylline concentrations in the 10 to 20llg/ml range. Transient intolerance is common when larger initial doses are used.

PI CM. 9yo WM Dx-Asthma wl32k

Ol . ____ 6m9/k9 load • . 5 Ol 1~ .

i ~ ~ 1:~ . I~'II ••• _ .. __ o o~ • I : f~"Cl 0 12 24 0 72 96 « .5 ..5 Hours of aminophylline therapy

Fig. 12. The effect of loading doses of theophylline (as intravenous aminophylline) followed by continuous infusions among 3 patients with strikingly different dosage requirements. .

Patient GR (middle) at a similar age and diagnosis to patient ES (left) exhibits much slower infusion dosage requirements and actually became toxic at a dose that maintained patient ES within the therapeutic range.

In contrast is a child (right) who required a dose more than 50% greater than that of patient ES in order to maintain a serum theophylline concentration within the therapeutic range. The patient also illustrates the value of repeated monitoring of the serum theophylline concentration and utilising loading doses to rapidly return the patient's serurn concentration to the therapeutic range once it has fallen (Weinberger et aI., 1976). .

Monitoring Serum Theophylline levels 306

Table II. Dose adjustment guided by serum theophylline concentration (adapted from Hendeles et aI., 1978b)

Peak theophy!!ine

level (pg/mll

< 5 5-7.5

8-10

11-13

14-20

21-25

26-30 31-35

;;. 35

Approximate

adjustment in

total daily dose

100% Increase'

50% increase

20 % increase

Cautious 10% increase if clinically indicated

None

Occasional intolerance requires a 10% decrease

10% decrease

25 % decrease 33 % decrease

50% decrease

To avoid potential toxicity:

Comment

If patieflt is asyml3tGmatic, consider trial off dwg'

repeat serum levels after adjustment

Even if patient is asymptomatic at this level, an in-

creased serum concentration may prevent symptoms during a viral URI, or heavy exposure to an inhalant allergen

If patient is asymptomatic, no increase is necessary. If

symptoms present during URI or exercise, increase as indicated

If 'breakthrough' in asthmatic symptoms present at end

of dosing interval, change to sustained release product

and repeat serum level

If side effects present, decrease total daily dose as

indicated

Even if side effects are absent

Even if side effects are absent, omit next dose and de­

crease total daily dose as indicated. Repeat serum levels

Omit next 2 doses, decrease as indicated and repeat

serum levels

(1) Assure that the sample represents a peak level obtained at steady state (e.g. no missed or extra doses with close approx­

im<!tion of prescribed dosing intervals during previous 48 hours) (2) Repeat laboratory determination if not initially performed in duplicate (3) The increase of 50 or 100% should be made in 25% increments at 2 day intervals to further assure safety and tolerance.

The final dose of theophylline for long term ther­apy is most readily determined by progressively in­creasing the dose from the mimimum starting level as described above, until the average dose for that age group is reached, if tolerated. Average doses are 24mg/kg/day for children up to 9 years of age, 20mg/kg/day for 9 to 12 year old children, 18mg/kg/ day for 12 to 16 year old children, and 900mg/dayforadults(fig. 7)[Wyattetal., 1978]. At these average doses, however, about 10 to 20 % of patients will have serum concentrations over 20}lg/ml and be at risk of toxicity. Therefore, these doses or higher should not be maintained without the assurance provided by peak serum theophylline

measurements less than 20}lg/ml, with appropriate dosage adjustment if necessary (table II). It is impor­tant that dosage adjustment be made cautiously and in small increments since dose dependent elimination kinetics have been demonstrated for theophylline and a relatively small change in dose might result in a dis­proportionally larger change in serum concentration (Weinberger and Ginchansky, 1977).

It is important that blood samples obtained for guidance of continued theophylline therapy be ob­tained when steady state conditions are present. If the patient has not had stable dosage for 3 days, a serum· measurement may not be valid for dosage adjust­ment. Missed or changed doses should not therefore

Monitoring Serum Theophylline Levels

have occurred during the 72 hOUl'S prior to obtaining the serum theophylline measurement. Under these stable conditions, dosage should be guided predomi­nantly by the peak serum theophylline concentration during a typical dosing interval. The time for samp­ling in order to obtain a reasonable approximation of the peak measurement is dependent upon the prepara­tion used. For solutions and solid dosage forms with rapid dissolution characteristics, 2 hours approxi­mates the peak, while 4 hours approximates the peak measurement with the slow release preparations Slo­Phyllin Gyrocaps, Theophyl S-R and Theo-Dur.

Measurement of trough blood levels (obtained at the end of a dosing interval) may provide additional useful information to guide the dosing interval or product selection, but should not be used alone to guide dosage; e.g. a peak of 18J.1g/ml with a trough of 8J.1g/ ml during continued therapy should not necessi­tate a dosage increase but may, if symptoms occur at the end of a dosing interval, warrant the same total daily dosage with shorter dosing intervals or use of a more slowly absorbed preparation. Either strategy would result in approximately the same mean serum concentration but with a lesser peak­trough differentiaL

Following the establishment of the dosage regimen that maintains serum concentrations within the therapeutic range, dosage requirements normally re­main stable for extended periods (Ginchansky and Weinberger, 1977). In children, however, growth effectively results in decreasing weight adjusted doses and repeat measurements of serum theophylline may be needed as frequently as every 6 months during periods of rapid groWth. Changes in physiological status such as the presence of prolonged fever, altera­tion of thyroid function, alterations in liver or car­diovascular function may all influence drug elimina­tion and warrant re-examination of serum theophylline concentrations. Additionally, cigarette smoking is associated with increased rates of theophylline elimination and higher dosage require­ments (Hunt et aI., 1976; Jenne et aI., 1975). Patients who successfully stop smoking following initiation of theophylline therapy therefore have an increased risk

307

of toxicity if dosage is not modified, since metabolic elimination of the drug decreases with resultant in­crease in serum theophylline concentrations.

Certain drugs also have a potential for altering theophylline elimination and thereby affecting the stability of serum theophylline concentrations during chronic dosage. Troleandomycin, a macrolide anti­biotic, has been shown to slow theophylline metabol­ism, and the related drug erythromycin has also been implicated in a drug interaction with accumulation of theophylline to toxic levels (see section I.~). Conver­sely, theophylline may inhibit phenytoin absorption when taken concurrently and result in decreased seizure control (Hendeles et aI., unpublished data).

4. Assay Methods

4.1 Solvent Extraction and Spectrophotometric Quantitation (modification [Matheson et ai., 1977] of the method of Schack and Waxler [ I 949] )

This method utilises the differential extraction of theophylline from related substance into an organic phase with further purification by back extraction into an alkaline aqueous phase. The modified method eliminates interference from drugs such as phenobar­bitone, tetracycline, and prochlorperazine. Caffeine does not coextract with theophylline and thus does not appear. Theobromine, however, does coextract and has a similar absorbance spectrum; theobromine­containing beverages must therefore be withheld for at least 24 hours before sampling.

Absorbance is determined at both 275 and 310 nanometers, and the difference is utilised to quantitate theophylline (theophylline has no significant absor­bance at 310 nanometers). Some other drugs (table III) may interfere by affecting absorbance either at 275, thus giving falsely high measurements for theophylline (e.g. frusemide/furosemide), or by in­creasing absorbance at 310 nanometers, thus result­ing in falsely low values (e.g. salicylic acid meta­bolites) [Matheson et aI., 1977).

Monitoring Serum' Theophylline ·levels .. __

Table III. Drug interference with the spectrophotometric (UV) assay (adapted after Matheson et al.,"977)

False elevation

Frusemide (furosemide)

Probenecid Theobromine' Phenylbutazone Sulphonamides Hyperuricaemia

Variable'

Chlorothiazide Aspirin' Warfarin

No interference3

Allopurinol 'AIIII'ieiliiA

Caffeine Phenytoin

, High concentrations in hot chocolate and tea. 2 May produce false elevated or artifactually low results

depending upon the serum concentrations. , 3 Allopurinol and phenytoin do not interfere in vitro. In

vivo interference from a metabolite has not been excluded.

4.2 Gas Liquid Chromatography (GLC)

308

derivatisation, although developments in column technology could make derivatisation unnecessary in the near future. In the experience of the author, GLC also requires greater instrumental skills on the part of the analyst than other methods. The GLC procedures now a.'lailable to the clinical laboratory are best suited to the batch analysis of 10 to I 5 samples.

4.3 High Pressure Cation Exchange Chromatography

A report by Thompson and Nagasawa in 1974 in­dicated that high-pressure cation exchange chromatography could be used to separate theophylline and its metabolites in body fluids. This methodology was subsequently utilised in the first clinically applicable method for measuring serum theophylline by high-pressure liquid chromatography (Weinberger and Chidsey, 1975), thereby offering a rapid simple alternative to the original solvent extrac-

Analysis by GLC requires that a' compound be· tion procedure with its problem of specificity and also volatile in order to be carried in the gas phase. offering advantages over the gas chromatographic Although theophylline can be detected directly by determination with its complex and technically de-chromatography, there are distinct advantages to the mandi~g extraction and derivatisation procedures.- --preparation of a derivative before chromatography. The major limiting factor to the use of this method Underivatised theophylline chromatographs with a is the lack of prepacked cation exchange -resin

- broad~ -uhsymmetrical--peak--on"--most --stationary--- CAminex-A-:.5-)-columns: These must·be packed .by the .. _, .. __ phases as a result of column absorption, which makes user. Once a working column is obtained, however, quantitation by peak-height difficult and interference their life span is very long, even with injections of from other compounds more likely. A number of. whole serum, provided 5cm disposable precolumns derivatisation procedures have been employed; those are used. Subsequent to the development of this which employ flame-ionisatio'n (Dechtiaruk et aI., methodology, however, the commercial availability 1975; Greeley, 1974; Johnson et aI., 1975; Perrier of improved prepacked reverse phase columns and Lear, 1976), nitrogen-phosphorus (Least et al., described in section 4.4 have made this latter method 1976; Lowry et aI., 1977) or electron capture detec- more popular, though it has never been as well ex-tors (Schwertner et a\., 1976) being superior to more amined for freedom from interfering substanCes and simple techniques (Kowblansky et aI., 1973). stability of the standard curve.

GLC methods for theophylline, because of the need for sample extraction and solvent evaporation, are not as simple to perform as liquid, chromato­graphic methods or immunoassays. They do, how­ever, exhibit good specificity and with suitable detec­tors are capable of micro-scale analysis. With the col­~mn materials now available, there is still need for

4.4 Reversed Phase Liquid Chromatographic .' Determination of Theophylline

Reversed-phase liquid chromatographic (RP-LC) determination of theophylline has achieved accept-

Monitoring Serum Theophylline Levels

ance in many clinical laboratories. The basis for this acceptance lies in simple sample preparation, good specificity, small sample volume, reasonable chroma­tography time, and the flexibility that RP-LC offers in quantitating other drugs such as anticonvulsants. The procedure has been discussed by Orcutt et al. (J 977), Adams et al. (1976) and Cooper et al. (1977).

Reverse-phase liquid chromatography is one of the best methods currently available for quantitation of theophylline. The simple sample preparation and small volume requirements make the method especially suitable for monitoring theophylline ther­apy in a paediatric population. Emergency determina­tions on single samples can be provided within 30 to 40 minutes.

4.5 Radioimmunoassay of Theophylline

Radioimmunoassay (RIA) of theophylline has been described by Cook et al. (1976) and Neese and Soyka (J 977). Both antibody preparations exhibited some cross-reactivity with caffeine and the major metabolites of theophylline. However, interference with either RIA would not be expected unless caffeine or theophylline metabolites were much greater in concentration than theophylline. Neese and Soyka (J 977) found good agreement between their radioim­munoassay ,and liquid- and gas-chromatographic determinations on a small number of sera to which theophylline had been added. Either RIA method could analyse sample volumes as small as 2 to 5}!l.

Both radioimmunoassay methods as described ap­pear more suited to routine therapeutic monitoring or pharmacokinetic studies in infants than to emergency determination of theophylline. Routine use of a theophylline radioimmunoassay in the clinical lab­oratory must await development of commercial kit procedures. 1 Theophylline labelled with a gamma

I A commercial radioimmunoassay procedure for theophylline which usesan "51 label is available from Clinical Assays, Cambridge, Massachusetts, 02139, USA.

309

emitter such as 1251 would be more useful than the tri­tium label and would be easier to automate.

4.6 Enzyme Immunoassay

Syva produces an enzyme immunoassay for theophylline based on competitive protein binding using an enzyme, glucose 6-phosphate dehydrogenase (G6PDH), as a label and an antibody as a specific binding protein (Syva, 1977). Serum or plasma is mixed with the reagent which contains antibodies to theophylline together with the substrate, nicotin­amide adenine dinucleotide (NAD), for the enzyme G6PDH. Binding of the antitheophylline antibody oc­curs to any theophylline in the serum or plasma. Theophylline labelled with G6PDH is then added. The labelled drug combined with any remaining un­fllied antibody binding sites. When the enzyme labelled theophylline becomes bound to antibody, the activity of the enzyme is reduced. The residual enzymatic activity is thereby directly related to the concentration of the drug present in the serum or plasma. The active enzyme converts the NAD to NADH, resulting in an absorbance change that is measured spectrophotometrically. Interference from serum G6PDH activity is avoided by use of the co­enzyme NAD that functions only with the bacterial (Leuconostoc mesenteroides) enzyme employed in the assay.

In addition to the reagents from Syva, this methodology requires a microsample spectrophoto­meter with a thermal control unit connected to a printer calculator and an appropriate pipette diluter for sample handling. Its sample requirements are 50}!1 per assay with duplicates required to assure ac­curacy.

Within day analysis reveals a coefficient of varia­tion less than 10 %, day-to-day analysis coefficient of variation was less than 15 %, and sample to sample analysis coefficient of variation was less than 10%. Comparison with various high pressure liquid chromatographic assays from 4 different laboratories revealed correlation coefficients ranging from 0.95 to

Monitoring Serum Theophylline Levels

0.98 with no consistent errors. The micropipetting procedures, however, result in occasional dilutional errors and duplicate determinations arc essential. Normally encountered levels of other xanthines or uric acids do not interfere with this assay.

This method offers advantages of specificity, rapidity and ease of operation. For a generallaborato­ry, it offers the additional advantage that the same equipment can be utilised for a variety of other drug assays with a full range of reagents available for anti­convulsants. While reagent costs are moderately high ($1. 7 5 per assay), equipment costs are acceptable ($7500) and often partially available in well equipped laboratories. The speed and ease of the assay, along with low maintenance for the equipment, should keep technician costs low. This assay will probably be the methodology of choice for most laboratories that have not already invested in high pressure liquid chromatographic equipment dedicated to theophylline measurement.

4.7 Measurerri'ent of Saliva Concentrations of Theophylline

The use of saliva to estimate serum theophylline has intrigued it number of investigators (see review by Danhof and Breimer, 1978). Reports of a relatively constant ratio of saliva to serum theophylline con­centrations have not been consistently supported by subsequent investigation (Hendeles et aI., 1977 d) and the routine clinical use of saliva as a substitute for serum or plasma cannot be recommended at this time for general use. Moreover, the early interest in saliv­ary levels occurred prior to the current micro­methodology and was also based on the assumption that large numbers of measurements might be of value to define kinetic characteristics of individual patients. For clinical purposes, however, relatively few (often only one) serum or plasma samples are needed to determine final long term theophylline dosage requirements (Hendeles et aI., 1978b). Even among children, small blood samples by venipuncture are generally well tolerated and well within the

310

capabilities of any clinician competent to manage chronic asthma. There is, therefore, both insufficient data and insufficient need to replace the established use of serum or plasma levels with estimates based on salivary measurement.

Acknowledgement

The authors wish to thank Dr Baer of the American Society of Clinical Pathologists for permission to base this article on material originally published by the American Society of Clinical Pathologists in their Technical Im­provement Service Bulletin.

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Monitoring Serum Theophylline Levels

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Author's address: Prof. Leslie Hendeles, College of Pharmacy, lJniversity of Iowa, ~owa City, Iowa 52242 (US,:\).