Cytotoxic Chemotherapy Michael B. Sawyer, MD, B.Sc. Phm.cyclophosphamide, ifosfamide 2) Other agents...

Cytotoxic Chemotherapy Michael B. Sawyer, MD, B.Sc. Phm.

1The screen versions of these slides have full details of copyright and acknowledgements

Cytotoxic Chemotherapy

1

Michael B. Sawyer, MD, B.Sc. Phm.Associate ProfessorUniversity of Alberta

Outline

• Alkylators

• Platinum compounds

T b li ti t

2

• Tubulin acting agents

• Antimetabolites

• Anthracyclines

• Topoisomerase inhibitors

Alkylating agents• Definition:

– Highly reactive chemicals that introduce alkyl radicals into biologically active molecules thereby preventing their proper functioning

• Attach alkyl group to DNA

• Act non-specifically

3

• Most common site of alkylation is the O6 position of guanine

• Cross link guanine nucleobases in DNA double-helix strands, thereby directly attacking DNA and not allowing DNA to uncoil and separate therefore not allowing cell division

• Some require conversion to active substances (e.g. cyclophosphamide)



Classification of alkylating agents

• Electrophilic

– Deliver the equivalent of an alkyl cation

– These are the antineoplastic agents

4

p g

• Nucleophilic

– Deliver the equivalent of an alkyl anion

• Radical

• Carbene

Alkylating agents

• Electrophilic side chains are either:

– Monofunctional - only react with one guanine residue

– Bifunctional - react with 2 different guanine residues

5

g

• Can form DNA interstrand cross-links, preventing DNA helix uncoiling if reacting to 2 different strands

• If guanine residues are on the same strand this doesn’t prevent the separation of DNA but does prevent DNA processing enzymes from accessing the DNA

Categories of alkylating agents

1) Nitrogen Mustards - melphalan, chlorambucil, cyclophosphamide, ifosfamide

2) Other agents - Chloroethyl Nitrosoureas (BCNU, CCNU) d b i b lf thi t t l id

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CCNU), dacarbazine, busulfan, thiotepa, temozolomide, lomustine, mitomycin C

3) Platinum compounds – cisplatin, carboplatin, oxaliplatin

4) Newer agents – trabectedin, others



Nitrogen mustards

• Bifunctional drugs with 2 electrophilic chloroethyl groups that alkylate DNA predominantly at the electron-rich N-7 position of guanine

• This reaction may proceed via the unimolecular ring i f th i i i ith f ti

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opening of the immonium ion with formation of a carbonium ion intermediate

R

Mechlorethamine (nitrogen mustard)• The first cancer chemotherapy drug; now used mainly to treat

Hodgkin's lymphoma as part of the MOPP regimen (combined Mechlorethamine-vincristine (Oncovin)-Procarbazine-Prednisone)

• Problems with clinical use because of its high reactivity

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• Because of its rapid reaction rate, mechlorethamine must be given i.v.

• As with all alkylating agents, bone marrow function is depressed

Melphalan (L-phenylalanine mustard)

• One of several modified mustards that were synthesized in an attempt to produce a drug that would preferentially

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in an attempt to produce a drug that would preferentially localize in a particular tumor

• Phenylalanine is a precursor of melanin; a phenylalanine mustard derivative might accumulate in melanomas and produce a selective toxicity

• Although this effect was not achieved, melphalan is a very useful drug



Melphalan (L-phenylalanine mustard) (2)

• The presence of the electron-withdrawing aromatic ring next to the nitrogen slows rates of cyclization (immonium ion formation) and alkylation, making melphalan much less reactive than mechlorethamine

• This allows time for absorption and wide distribution before

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pextensive alkylation occurs

• It also means that melphalan can be given orally

• Melphalan crosslinks DNA in a manner similar to mechlorethamine (guanine’s n-7 position)

• Used to treat multiple myeloma; also ovarian/breast tumors

• Major toxicity is bone marrow suppression

Cyclophosphamide/Ifosfamide

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• Another "modified" mustard developed in the hope that might be preferentially activated in tumor cells

• High phosphoramidase and phosphatase activity had been seen in some tumors; cleavage of the phosphamide ring might produce an active compound within tumor cells

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Cyclophosphamide/Ifosfamide (2)

• Although tumor activation was not achieved, cyclophosphamide has wide applications

• Nontoxic pro-drug that is metabolically activated to 4-hydroxycyclophosphamide by hepatic microsomal cytochrome P 450

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cytochrome P-450

• Given either orally or i.v.

• Used to treat CLL, multiple myeloma, Hodgkin's disease, non-Hodgkin's lymphomas, ALL, Burkitt's lymphoma, and a variety of solid tumors, including Ewing's sarcoma, rhabdomyosarcoma, neuroblastoma, and carcinomas of the breast, lung, and ovary



Cyclophos/Ifos

• “Phosphoramide mustard” is probably the actual alkylating agent (bifunctional) at guanine N-7; this causes the CNS toxicities from ifos

• Acrolein causes hemorrhagic cystitis(MESNA neutralizes through sulfhydryl group)

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(MESNA, neutralizes through sulfhydryl group)

• Patients need high amounts of fluids to prevent cyclophosphamide-induced hemorrhagic cystitis

• Most widely used agents of the bis-chloroethyl type; has a broad clinical spectrum of activity and is an essential component of multidrug combinations

Chloroethyl nitrosoureas

• The 2-chloroethyl carbonium ion (+CH2CH2Cl)is the alkylating moiety of these drugs

Carmustine(BCNU)

Lomustine(CCNU)

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of these drugs

• BCNU has 2 chloroethyl groups, whereas CCNU and methyl-CCNU are monofunctional agents; BCNU induces interstrand crosslinks

Semustine(Methyl-CCNU)

Chloroethyl nitrosoureas (2)

• Nitrosoureas are highly lipophilic and readily pass into the cerebrospinal fluid therefore used to treat CNS tumors

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• Significant activity in Hodgkin's disease and melanoma

• BCNU is given i.v.; CCNU and methyl-CCNU are given orally



Dacarbazine (DTIC)

• Structural analogue of imidazole carboxamide, a purine precursor

• Undergoes activation via hepatic cytochrome P450 to the active compound, methyltriazenoimidazole carboxamide (MTIC)

• MTIC cytotoxicity is thought to be due primarily to methylation of DNA at the guanine’s O6 position

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of DNA at the guanine s O6 position

• May also inhibit DNA and RNA synthesis by acting as a purine analogue and by interacting with sulfhydryl groups

• cell cycle phase-nonspecific and is mildly immunosuppressive

• Metastatic melanoma (single agent), Hodgkin’s (ABVD), sarcoma (MAID)

• Major toxicities: nausea, vomiting, leukopenia, thrombocytopenia

Temozolomide

• Temozolomide does not require metabolic activation by the cytochrome P450 (unlike dacarbazine which does)

• Undergoes rapid chemical conversion at physiologic pH to the active compound, MTIC

f

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• Antitumour activity of temozolomide is schedule dependent; compressing the schedule may allow subsequent doses of temozolomide to be given when levels of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) are low, thereby prolonging systemic exposure to the drug and MTIC, to improve cytotoxicity and response rate

Mitomycin C

• Derived from Streptomyces caespitosus

• Selectively inhibits DNA synthesis by causing cross-linking, degrades preformed DNA,

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and causes nuclear lysis and formation of giant cells

• At high concentrations, cellular RNA and protein synthesis may be suppressed

• Cell cycle phase-nonspecific, although it has its maximum effect in late G- and early S-phases



Trabectedin• Ecteinascidin 743 or ET-743, Trabectedin

• Isolated from the sea squirt Ecteinascidia turbinata

• Interacts with the minor groove of DNA and alkylates guanine at the N2 position

• Affects various transcription factors involved in cell proliferation,

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particularly via the transcription-coupled nucleotide excision repair system

• Blocks cell cycle at the G2 phase; cells at the G1 phase are most sensitive to the drug

• Granted orphan drug status by the FDA for recurrent soft tissue sarcoma in October 2004

• Major toxicity is neutropenia and transaminitis

Cisplatin

• Definition of serendipity

• Barnett Rosenberg at Michigan State University studying effects of electrical fields on bacterial growth

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• Noted that the bacteria could not divide and became filamentous

• Found platinum was leaching off the platinum electrode and forming chemically reactive species

Cisplatin (2)

• All active platinum drugs are in the Pt(II) state

• Can be cis or trans but trans are much less potent

• Binding RNA > DNA > protein

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Cisplatin (3)

• Mechanism of action is binding to DNA and possibly proteins

• Binds to purine bases especially

Preferred target is the 7 position nitrogen on adenine

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• Preferred target is the 7 position nitrogen on adenine or guanine

• Can bind to DNA in two ways

– Adduct intrastrand binding

– Cross link interstrand binding

Cisplatin (4)

• In vitro experiments of cisplatin DNA binding

– N7-d(GpG) 60% of binding

– N7-d(ApG) 30% of binding

– N7-d(GpXpG) 10% of binding

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– N7-d(GpXpG) 10% of binding

– N7-d(X)-d(X) 2% of binding

• X can be any deoyxnucleoside

24Rabik and Dolan, Cancer Treat Rev. 2007 33(1):9-23



Nucleotide excision repair

• Major mechanism of resistance to platinum

• Major protein complex involved in repair is excision repair cross complementation group 1 (ERCC1)

• First step is recruitment of XPA and RPA

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• Recognition of damage recruits endonucleases XPG and XPF ERCC1 which causes incisions at the 3’ and 5’ ends of the damaged DNA

• In vitro studies of testicular cancer show deficiency of ERCC1 and xeroderma pigmentosum complementation group F

ERCC1

Preincision complex

XPF

DNA i l ti dd t

ERCC1

Complex bound

Damaged DNA binding factor

DNA adduct

26Copyright ©2008 American Association for Cancer Research

Martin, L. P. et al., Clin Cancer Res2008;14:1291-1295

CCR molecular pathways

DNA polymerase

DNA cisplatin adduct

High mobility group proteins

• High mobility group domains contain 3 alpha helicies and are made up of 80 amino acids

• Bind to 1,2-(GpG) intrastrand cross links

Pre ent recognition b NER

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• Prevent recognition by NER

• Block replicative bypass

• Interestingly testis has a lot of proteins with mobility group domains



• Comprised in hMLH1, hMLH2, hPMS2, hMSH2, hMSH3 and hMSH6

• hMutSα (a heterodimer of hMSH2 and hMSH6)

Mismatch repair

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( )or hMutSβ (a heterodimer of hMSH2 and hMSH3) recognize the damage

• Recruit hMutLα (a heterodimer of hMLH1 and hPMS2)

Mismatch repair (2)

Mut protein

Mut protein

MismatchMut protein


Martin, L. P. et al., Clin Cancer Res2008;14:1291-1295

CCR molecular pathways

DNA polymerase

pcomplex

Mismatch repair (3)

• Only a two fold change in MMR can lead to cisplatin or carboplatin resistance

• Oxaliplatin adducts are NOT recognized by MMR

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• 10 to 20% of colon tumors have microsatellite instability either inherited or sporadic



Platinum uptake• Early researchers thought/assumed that cisplatin

and carboplatin diffused across the cell membrane

• But poisoning the sodium potassium ATPase with oubain blocked cisplatin uptake

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• Howell’s group at San Diego showed that pharmacologic cisplatin concentrations blocked copper transport

• Downregulated copper transporter 1 Ctr1

• The small cell lung cancer cell line SR2 which is platinum resistant had ½ the Ctr1 as its platinum sensitive counterpart

Cisplatin efflux transporters

• Two efflux transporters ATP7A and ATP7B

• Both responsible for transport of copper from the golgi to plasma membrane

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p

• Transfection studies showed a 60% decrease in cisplatin concentrations

Copper transporters

• Nakayama et al. profiled 82 ovarian cancers for various genes including the transports ATP7B, MDR1, MRP1, MRP2, LRP, BCRP

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• Only ATP7B was significant

• Can sequester all three platinums cisplatin, carboplatin, and oxaliplatin



Cisplatin vs. Carboplatin vs. Oxaliplatin

• Cisplatin least myelosuppressive

• Oxaliplatin more severe thrombocytopenia

N th i li >> b l ti

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• Neuropathy cis = oxali >> carboplatin

• Nephrotoxicity cisplatin >> oxali ~ carbo

• Oxaliplatin cold induced neuropathy

Neuropathy

• Predominantly sensory

• Probably basis in anatomy

– Dorsal root ganglion lies outside blood brain barrier

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g g

– High accumulations in platinum in animal studies

• Mechanism is unclear

Oxaliplatin neuropathy• Cold induced neuropathy usually last 3 – 5 days

after treatment

• Acute laryngeal dysfunction

– Treatment with ativan

• Possible treatment of neuropathy with calcium/

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Possible treatment of neuropathy with calcium/ magnesium infusion and glutamine

• Oxaliplatin metabolite is oxalate

• Possibly oxalate interferes with voltage gated sodium channels

• In ethylene glycol poisoning oxalate is believed responsible for the neurotoxicity



Tubulin/microtubule structure and function

• Microtubules

– Comprise of 13 linear protofilaments of heterodimers of

• α-tubulin

• β tubulin

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• β-tubulin

– Provide the following of functions:

• Cell division

• Directional transport of vesicles and organelles

• Signaling

• Cell shape and polarity

Tubulin/microtubule structure and function (2)

38Clin Cancer Res (2008) 14:7167-7172

Tubulin/microtubule structure and function (3)• Microtubule formation is a dynamic process:

– Dynamic instability:• Rapid switch of the end of microtubules between slow

sustained growth and rapid shortening

– Treadmilling:Pl s end ne heterodimers are added rapidl

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• Plus end: new heterodimers are added rapidly– GTP is hydrolyzed to GDP, which is required for net

polymerization of free tubulins

• Minus end: heterodimers are disassembled– Attached to the centromere, where there is minimal

assembly and disassembly of tubulins

• Any disruption of this assembly and disassembly of tubulins will lead to apoptosis



Targeting tubulins

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Vinca alkaloids• Origin:

– Vincristine and vinblastine are natural products from periwinkle

– The others are semi-synthetic

• Binding to β-tubulin at distinct site, particularly type II

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g β , p y yp

– Blocking polymerization by treadmilling but lesser effect on dynamic instability

– Delay in transition to M-phase and failure to progress to anaphase, leading apoptosis

• Uptake by diffusion

• Radiosensitizing

Vinca alkaloids (2)

PoorPoorGoodPoorCNS P t ti

0.6400.4-1.290.740.16Clearance (L/h/kg)

Shortest (~40h)

Shortest (18-49 h)

Intermediate (20-64 h)

Longest (23-85 h)Terminal t1/2

VinflunineVinorelbineVinblastineVincristine

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53%80-90%75-90%48-75%Protein binding

Very lowLowIntermediateHighest Neurotoxicity

NeutropeniaNeutropeniaNeutropenia

Peripheral neuropathy;

Paralytic ileus (axonal

degeneration)

Dose-limiting toxicity

Penetration



Vinca alkaloids (3)

• Rare toxicities:

– SIADH: Vincristine and vinblastine

– Cardiac:

• Hypotension or hypertension: Vincristine due

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Hypotension or hypertension: Vincristine due to autonomic dysfunction, but only hypertension from vinblastine

• Cardiac ischemia: vincristine > vinblastine

• Raynaud’s: vinblastine

• Non-cardiac chest pain: vinorelbine

• Mechanism of apoptosis: probably:– Paclitaxel: p53-dependent and independent mechanism– Docetaxel: p27 related– Both phosphorylation of bcl-2

• Inhibition of angiogenesis• Poor oral bioavailability secondary to high expression

of P-gp in GI epithelial cells

Taxanes

• Origin:– Paclitaxel: bark of Pacific yew– Docetaxel: Needles of European yew

• Binding to β-tubulin:– Particularly high affinity to type II, but not type III– Distinct site from vincas

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– Low concentration: suppresses dynamic instability at plus end, leading to decrease in polymerization

– High concentration: induced polymerization in the absence of GTP, MAP etc.• Radiosensitizing

• Metabolized by liver and eliminated by biliary• Pharmacokinetic profile:

– Large volume of distribution (Vd)– High Clearance (CL)– Long terminal half-life

• Given 5-10% of paclitaxel and 1-5% of docetaxelis renally cleared, so cisplatin should be given after paclitaxel and possibly docetaxel; otherwise, will increase AUC and thus toxicity from taxanes

Taxanes (2)

Non-linear (saturable distribution and elimination

Polysorbate 80Cremphor ELSolvent

1 hr weekly or q3wkWeekly 1-h

3h or 24h q3wkSchedule

DocetaxelPaclitaxel

45Neutropenia

Neutropenia and peripheral sensory neuropathy

Dose-limiting toxicity

CYP 3A4/5, 2B, 1ACYP2C8; 3A4Metabolism

>90%95%Protein binding

Linear

Triphasic

distribution and elimination, esp. short infusion)

Biphasic for 24-h; triphasic for 3-h

Pharmacokinetic profile



Taxanes (3)• Toxicities of Paclitaxel:

– ANC nadir at day 8-15, depending on the duration of the plasma concentration above 0.05-0.1 micromolar(24-h >3h)

– Hypersensitivity reaction: AUC of Cremphor EL

– Neuropathy:

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• Sensory > motor, due to axonal degeneration

• Proportional to AUC of Cremphor EL

– Myalgia/athralgia: proportional to AUC or Css of paclitaxel

– Cardiac:

• Arrhythmia: mechanism unknown

• CHF: particularly with herceptin and doxorubicin

Novel taxanes• New formulation:

– Nab-Paclitaxel: nanopoarticles collidal suspension of paclitaxel in 3-4% albumin

• Advantages:

– Solvent free, therefore no hypersensitivity reaction

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– Longer half-life and Vd

– Increase plasma free paclitaxel and thus intra-tumoural accumulation and clinical efficacy

– Decrease in neutropenia (9 vs. 22%) compared to paclitaxel

– Increase in grade 3 sensory peripheral neuropathy (10 vs. 2%)

• Origin: fermentation by myxobacterium

• Binding to β-tubulin

– At the same or neighbouring region as that of taxanes

L di t l i ti i t i t b l d th

Epothilones

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– Leading to polymerization into micortubules and thus G2/M arrest

– Affinity:• Epothilone A = paclitaxel

• Epothilone B > paclitaxel



Epothilones (2)

Drug accumulation: No

Half-life: 29-49 hr

Drug accumulation 2X

Half-life: 4 days

Cremphor EL, need premedication with

H1/H2 blockersNoSolvent

IxabepilonePatupilone

49Unknown, but unlikelyYesCNS penetration

CYP3A4CYP3A4Metabolism

Sensory neuropathy

NeutropeniaDiarrheaDose-limiting toxicity

Large Vd >1000L

Linear PK

Short infusion has increased toxicities

Large Vd >1000L

Linear PK

No difference between short and long infusion

Pharmacokinetics

Antimetabolites• All agents of this class interfere with DNA replication

or nucleotide synthesis

• Interfere with nucleotide synthesis: antifolates

– Methotrexate

– Raltitrexed

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– Pemetrexed

• False analogues

– 5-flurouracil

– Cytarabine (Ara-C)

– Gemcitabine

– Fludarabine

– Capecitabine

Antifolates

• Second oldest class of chemotherapy

• 1948 Farber S and Diamond LK “Temporary remissions in acute leukemia in children produced by folic acid

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p yantagonist, 4-aminopteroyl-glutamic acid” NEJM

• Complete serendipity

• “Acceleration factor” and folic acid



Chemical structure

N

NN

N

O H

NH

H N

O

O

O H

OH O

H 2N

510

(Folic Acid)2-Amino-6-((p-((1,3-dicarboxypropyl)carbamoyl)anilino)methyl)-4-pteridinol

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N

NN

N

N H2

N

H N

O

O

O H

OH O

H2

N

51 0

CH3

(Folic Acid)

4-Amino-10-Methylfolic acid (Methotrexate)

Modern antifolates

• Methotrexate

– ALL, primary CNS lymphoma, breast cancer, bladder cancer

Raltitre ed

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• Raltitrexed

– Colon cancer

• Pemetrexed

– Malignant mesothelioma, non-small cell lung cancer

Basic pharmacology

• Small molecule MW around 400, very hydrophillic

• Liver metabolism very minimal

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• Biliary excretion futile

• Renal elimination is dominant



Pemetrexed• Newest antifolate

• Polyglutamated forms target the same enzymes as methotrexate– Thymidylate synthase

– Dihydrofolate reductase

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– Glycinamide ribonucleotide formyltransferase

– Aminoimidazole carboxamide ribonucleotide formyltransferase

• Pemetrexed has a higher affinity for folypolyglutamate synthetase (FPGS) than MTX

• Pemetrexed and raltitrexed are better ligands for alpha folate receptors

Basic pharmacology

• Two carrier proteins

– Reduced folate carrier

– Folate receptor alpha, beta and gamma

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• Reduced folate carrier ubiquitous

• Folate receptor more specialized and localized

– Renal

– Intestinal

• Trapped in cells by polyglutamation

Renal dysfunction

• Most feared antifolate toxicity is renal

• Method of elimination is renal

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• Renal dysfunction secondary to methotrexate is a life threatening problem



Risk factors

• Vomiting/Diarrhea

• Fever

• Third space (pleural effusions/ascites)

NSAID

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• NSAIDs

• Penicillins/cephalosporins

• Probenecid

• Acidic urine

5FU pharmacology

• Half-life is 7 to 20 minutes

• Bolus administration predominately effects on RNA and protein synthesis

• Continuous infusion more DNA effects

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• Continuous infusion more DNA effects

• Transporters poorly understood, only well defined is hENT2

• Can be administered in relatively extreme liver and kidney dysfunction

Nucleosides

• Pyrimidine

– Azacitidine, cytarabine, capecitabine, floxuridine, gemcitabine

• Purine

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Purine

– Cladribine, clofarabine, fludarabine, pentostatin



Capecitabine

• Technically speaking not a nucleoside

• Prodrug with a blocking group that allows oral absorption

• Carboxylesterase activates the drug

61

y g

• Thymidine phosphorylase final activation step

Cytarabine

• Originally isolated from Cryptothethya crypta

• Inhibits DNA polymerase alpha and at high concentrations beta

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• Incorporation into DNA blocks chain elongation leading to chain termination

• Inhibition of ribonucleotide reductase

Gemcitabine

• dFdCTP inhibits DNA polymerase

• Potently inhibits ribonucleotide reductase which decreases deoxyribonucleotide pools

• One more nucleotide is added to the lengthening chain

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• One more nucleotide is added to the lengthening chain

• Depletion of dCTP decreased cytidine deaminase activity

• Longer intracellular retention t1/2 alpha 3.9 hrs and beta 16 hrs vs. cytarabine 0.7 hr



Purine analogues

• “Boy in the Bubble”

• Adenosine deaminase deficiency

• Toxic to lymphocytes

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• Development of analogues to inhibit ADA

Fludarabine

• Poorly water soluble, solubilized as the monophosphate

• Inhibits DNA polymerase, DNA ligase, DNA primase, and ribonucleotide reductase

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• Incorporated into DNA and causes chain termination

Nucleoside transporters

• Equilbrative Nuceloside Transporters

– hENT1 NMBPR

– hENT2 Dilazep, dipyridamole

• Concentrative Nucleoside Transporters

66

p

– hCNT1 pyrimidine

– hCNT2 purine

– hCNT3 broadly selective



Fig. 2. Kaplan-Meier estimate of survival in gemcitabine-treated pancreatic cancer patients. Patients for whom all adenocarcinoma cells had detectable hENT1 (□ and dashed line) had significantly longer survival than those patients with heterogeneous areas of adenocarcinoma cells lacking hENT1 (○ and continuous line; median survival 13 versus 4 months; P = 0.01)


Spratlin, J. et al., Clin Cancer Res 2004;10:6956-6961

Definition of topoisomerases

• Key enzymes controlling the changes in DNA structure by catalyzing the breaking and rejoining of the DNA phosphodiesterase backbone during the normal cell cycle

• Amount of supercoiling of DNA helix is regulated

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• Amount of supercoiling of DNA helix is regulated by topoisomerases

• Allows DNA relaxation for replication

– Reduces DNA twisting and super-coiling that occur due to replication, transcription and other nuclear processes

• Enzymatic mechanism involves sequential transesterifications

Classes of topoisomerases

• Two main types based on ability to cleave 1 or 2 strands of DNA:

– Type I: monomers, relieve “+” and “–” supercoils produced through fork movement during transcription; cleaves single DNA t d d i bili it it ti

69

DNA strand and immobilizes it so it can continue to replicate without strain

• IA: not in humans

• IB: found in humans, relaxes “+”vely supercoiled DNA

– Type II: dimeric, catalyze breakage of both DNA strands, relaxes DNA supercoils formed during transcription and other cellular processes; require Mg and binding of ATP



Topoisomerase inhibitors

• Interfere with the action of topoisomerases (either I or II)

• Block ligation step of cell cycle generating single or double strand breaks

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or double strand breaks

• Lead to apoptosis and cell death

Topoisomerase inhibitors (2)

• Thought to bind to DNA or topoisomerase near the DNA-protein linkage

• Can be classified as:

– Catalytic vs. poisons

71

• Catalytic inhibit enzymes that cleave DNA; poisons stabilize fragile and transient cleavage complexes and prevent strand re-ligation

– Intercalating vs. nonintercalating

• Depending on whether they interact with enzyme and DNA forming stable covalent complexes known as a “cleavage complex”

Classes of TI

• Topoisomerase I Inhibitors:

– Camptothecin

– Irinotecan (camptothecin analog)

– Topotecan (camptothecin analog)

72

• Topoisomerase II Inhibitors:

– Etoposide

– Tenoposide

– Anthracyclines, Mitoxantrone

• Dual I and II Inhibitors



Topoisomerase I inhibitors

• Interact with Top1-DNA aggregate to form a cleavable complex preventing resealing of the Top1-induced DNA single-strand breaks by the reparative process

• Cytotoxicity highly dependent on DNA synthesis so most

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• Cytotoxicity highly dependent on DNA synthesis so most active during S phase

• Inhibit DNA synthesis leading to cell cycle arrest in G2 and cell death by apoptosis

Camptothecin

• Also called CPT

• First isolated in 1966 from Camptotheca acuminata, tree native to China

74

• Therapeutic application hindered by low solubility, high toxicity, rapid inactivation at physiologic pH

Irinotecan

• Aka, CPT-11 or camptosar

• Semisynthetic, water-soluble derivative of camptothecin

• Cytotoxic alkaloid extracted from plants such as Camptotheca acuminata

75

• Active metabolite, SN-38

• Irinotecan and SN-38 bind to the topoisomerase I-DNA complex and prevent religation of the DNA strand, resulting in double-strand DNA breakage and cell death

• Cell cycle phase-specific (S-phase)



Irinotecan special precautions

• Prior radiation

• Hyperbilirubinemia

• Gilbert’s syndrome

76

• Gilbert s syndrome

• Renal dysfunction

• Elderly

Topotecan

• Water-soluble derivative of camptothecin

• Alkaloid extracted from plants such as Camptotheca acuminata

• Topotecan is found predominantly in the inactive

77

• Topotecan is found predominantly in the inactive carboxylate form at neutral pH and it is not a prodrug

• Radiation-sensitizing agent

• Cell cycle phase-specific (S-phase)

Topotecan special precautions

• Renal dysfunction (dose adjust for CrCl below normal and don’t give if < 20mL/min)

• Apparently safe in bili up to 170microM/L

78

• ? Carcinogenic

• Neutropenia, thrombocytopenia most common toxicities



Topoisomerase II inhibitors

• Interact to form a double strand break

• More Top2 in proliferating cells therefore toxic effects of targeting this enzyme more pronounced

79

in neoplastic tissue

Etoposide

• Semisynthetic derivative of the podophyllotoxins, an epipodophyllotoxin

• Cell cycle dependent and phase specific, affecting mainly the S and G2 phases

80

y p

• Is orally bioavailable

• Crosses blood brain barrier

Etoposide special precautions

• Potentially carcinogenic

• Neutropenia, thrombocytopenia (recovery around d20), fatigue, alopecia, anorexia, nausea, vomiting, mucositis = main common toxicities

81

• CHF/MI

• Allergic reaction (esp i.v.)



Tenoposide

• Semisynthetic podophyllotoxin derived from the root of Podophyllum peltatum (the May apple or mandrake)

• Analog of etoposide

• Cell cycle phase-specific with predominant activity

82

• Cell cycle phase-specific with predominant activity occurring in late S phase and G2

• High concentrations in liver, kidneys, small intestines and adrenals; found in ascitic fluid

• Crosses blood brain barrier

Tenoposide special precautions

• Contraindicated in patients who have a history of severe hypersensitivity reactions to teniposide or other drugs formulated in Cremophor EL

• Potentially carcinogenic

83

y g

• Extravasation hazard

• Same general toxicities as etoposide

Anthracyclines

• All anthracyclines have a tetracycline ring structure attached by a glycosidic bond to a sugar daunosamine

• Four anthracyclines are currently in the clinic– Doxorubicin

84

Doxorubicin

– Epirubicin

– Daunorubicin

– Idarubicin

• Doxorubicin was originally isolated from the fungus Streptococcus peucetius and the protypical anthracycline



Anthracycline mechanism of action

• Bind to DNA and topoisomerase II leading to double strand DNA breaks

• Intercalates between DNA bases and undergoes redox cycling leading to free radicals

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y g g

• Bind to cell membranes leading to lipid peroxidation

• Above mechanisms lead to apoptosis

Anthracycline pharmacokinetics

• Elimination is predominately hepatic through aldo keto reductases for doxorubicin and daunorubicin

• Although epirubicin is the 4’-epimer of doxorubicin it undergoes substantially different metabolism

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g ywith its dominant path of elimination being glucuronidation by uridine glucuronosyl transferase 2B7

• The anthracyclines are extensively protein bound and have very high volumes of distribution

Anthracycline toxicities

• Myelosuppresion is common

• Stomatitis, gastrointestinal disturbances such as nausea/vomiting and diarrhea are common

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• Usually cause complete hair loss

• Major chronic toxicity is cardiomyopathy



Anthracycline Cardiotoxicity

• Acute form

– ECG abnormalities and arrhythmias including ventricular tachycardia

– Severe form is pericarditis-myocarditis

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p y

• Chronic form (more common)

– Cumulative dose at 550 mg/m2 is associated with a 20% risk

– Generally considered irreversible

– Dexrazoxane can protect against

The End

Thank You

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Thank You

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Cytotoxic Chemotherapy Michael B. Sawyer, MD, B.Sc. Phm.cyclophosphamide, ifosfamide 2) Other agents...

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Transcript of Cytotoxic Chemotherapy Michael B. Sawyer, MD, B.Sc. Phm.cyclophosphamide, ifosfamide 2) Other agents...