Anti-Retroviral Agents (ARVs)

BY; SEYOUM GIZACHEW (B.Pharm., MSc)

2

Anti-Retroviral Agents (ARVs)

HIV-1 Virology

3

The HIV Epidemic Unfolds

• Sudden outbreak in USA of opportunistic infections and cancers in homosexual men in 1981.– Pneumocystis carinii pneumonia (PCP), Kaposi’s

sarcoma, and non-Hodgkins lymphoma • HIV isolated in 1984 - Luc Montanier (Pasteur Institute,

Paris) and Robert Gallo (NIH, Bethesda, USA) • HIV diagnostic tests developed in 1985.• First antiretroviral drug, zidovudine, developed in 1986 • Exploding pandemic.– Has infected more than 50 million people around the

world – Has killed over 22 million people.

4

Classification of HIV

• Retrovirus: single stranded RNA transcribed to double stranded DNA by reverse transcriptase

• Integrates into host genome• Can lie dormant within a cell for many years, especially in

resting (memory) CD4+ T4 lymphocytes • HIV type (distinguished genetically) – HIV-1 -> worldwide pandemic (~ 40 M people)– HIV-2 -> isolated in West Africa; causes AIDS much more

slowly than HIV-1 but otherwise clinically similar.

5

6

7

How HIV Enters Cells

• There are three crucial steps for entry of HIV into the CD4 T cell:

1.Binding of HIV via the gp120 envelope protein to the CD4 receptor (attachment),

2.Binding to coreceptors via conformational changes to gp120, and finally

3.Fusion of virus and cell• In addition to the CD4 receptor, HIV requires coreceptors for

entry into the target cell. • The two most important ones are CXCR4 and CCR5. • HIV variants mainly using CCR5 are referred to as R5

viruses; those using CXCR4 are referred to as X4 viruses.

8

How HIV Enters Cells cont…

• When the virus enters the blood, it binds a protein on its surface (gp120) to a CD4 receptor and co-receptors (CXCR4 or CCR5) on the CD4 cell. • Then gp120 shifts to expose gp41. • Once exposed, gp41 pierces the CD4 cell and pulls it in close

enough to allow viral-cell fusion.• Binding of virus to cell surface results in fusion of viral

envelope with cell membrane.• Viral core is released into cell cytoplasm.

9

HIV and Cellular Receptors

10

Viral-host Dynamics

• About 1010 (10 billion) virions are produced daily • Average life-span of an HIV virion in plasma is ~6 hours • Average life-span of an HIV-infected CD4 lymphocytes is

~1.6 days• HIV can lie dormant within a cell for many years,

especially in resting (memory) CD4 cells, unlike other retroviruses.

• The extremely high rates of viral replication results in every possible point mutation in the viral genome arising daily.

• In any given patient, the virus usually varies by 1-6% in the env gene, for example.

11

12

HIV Life Cycle and Sites for Therapeutic Intervention

13

Classes of Antiretrovirals• Fusion inhibitors– Prevents fusion of the virus into a CD4 cell by preventing

conformational change needed to allow virus to enter a CD4 cell

• Nucleoside reverse transcriptase inhibitors (NRTIs) or nukes (zidovudine, lamivudine)– Mimic naturally occurring nucleosides– Blocks viral DNA construction as they deceive reverse

transcriptase.• Non- nucleoside reverse transcriptase inhibitors (NNRTIs) or

non-nukes (Nevirapine, Efavirenz)– Bind to the reverse transcriptase enzyme

• Protease inhibitors (PIs) (Indinavir, lopinavir)– Prevents cleavage of precursor protein chains.

14

Nucleoside/Nucleotide Reverse Transcriptase Inhibitors (NRTI’s)

• Lamivudine (3TC)• Stavudine (d4T)• Zidovudine (ZDV, AZT)• Didanosine (ddI)• Tenofovir (TDF)• Abacavir(ABC)• Emtricitabine(FTC)• Zalcitabine(DDC)

15

NRTI Mechanism of Action

• NRTIs must first undergo intracellular phosphorylation to be active.

• NRTIs inhibit the viral reverse transcriptase enzyme– Enzyme responsible for transcribing viral RNA into

double stranded DNA• NRTIs mimic other nucleosides and are incorporated into the

DNA strand• They prevent the addition of the natural nucleosides into the

DNA strand• This halts the production of new virions

16

NRTI Class Side Effects• Nausea• Headache• Peripheral Neuropathy (DDC > d4T/ddI)• Lipoatrophy• Pancreatitis (ddI > d4T)• Lactic Acidosis– d4T > ddI > ZDV– Rare with ABC, TDF, 3TC and FTC

17

Zidovudine (ZDV)

• It is a deoxythymidine analog. • Phosphorylated by cellular thymidine kinase.• incorporated into DNA chain and terminates synthesis.• well absorbed from the gut and distributed to most body

tissues and fluids, including the cerebrospinal fluid• The most common adverse effect is myelosuppression,

resulting in macrocytic anemia.• Food decreases ZDV-related nausea• Pregnancy: category C • Advocated for pregnant women beyond first trimester to

prevent vertical transmission.

18

Zidovudine (ZDV) cont…

• Elimination: Metabolized by liver to 5’-glucuronyl zidovudine, which is renally excreted. – Reduce dose with renal or hepatic compromise:

• Drug Interactions: ZDV and D4T compete for activation by thymidine kinase.

• Combinations containing AZT plus d4T, is therefore pointless, since both drugs would compete for the same bases.

• The combination of these drugs is antagonistic in vitro and in vivo: do not use together.

19

Zidovudine (ZDV) cont…• Toxicity– Nausea– Bone Marrow Suppression• Anemia (fatigue)• Granulocytopenia (low platelet count)• Neutropenia (low white blood cell [neutrophils] count)

– Headaches – Myalgia– Myopathy – Insomnia– Pigmentation of nail beds– Lactic acidosis

20

Lamivudine (3TC)

• A synthetic nucleoside (cytidine) analogue reverse transcriptase inhibitor

• Active against HIV-1, HIV-2, and hepatitis B virus.• Phosphorylated 3 times by thymidine kinase to the active

metabolite, 3TC-triphosphate. • Competes with deoxycytidine triphosphate (dCTP) for

binding to reverse transcriptase and incorporation of the nucleoside analogue into viral DNA results in the termination.

• 3TC-TP is a weak inhibitor of mammalian DNA polymerases α, β, and γ.

• Bioavailability (F): 86%.

21

Lamivudine (3TC) cont…

• CSF Levels: 13% of serum levels (these levels have been shown to clear HIV RNA from CSF)

• T1/2: 3-6 hours• Intracellular T1/2: 12 hours• Pregnancy: (Category C) – Crosses placenta. – Use in pregnancy is safe and well established, and

effective. – When combined with Zidovudine, efficacy in preventing

perinatal transmission is also well established. • rapid development of resistance, and a single point mutation

on RT gene.

22

Lamivudine (3TC) cont…

• Intracellular T1/2 – NRTIs work in the cell, duration of action is based on

intracellular half-life.• Toxicity: very rare– Headache– Occasional nausea– Lactic acidosis

23

Tenofovir Disoproxil Fumarate (TDF)• TDF is different from nucleosides in that it has already been

phosphorylated once.• T1/2: 12 to 18 hours• Intracellular T1/2: 10 to 50 hours• Pregnancy: (Category B) Crosses placenta. • Bioavailability improves with food, especially high-fat meal.• Primarily eliminated by a combination of glomerular

filtration and active renal tubular secretion. • Toxicity– Headache, Nausea, diarrhea– Lactic acidosis, fatty liver– Renal insufficiency (rare)

24

NRTI Mitochondrial Toxicity

• Inhibition of mitochondrial DNA polymerase-– oxidative metabolism, – ATP generation

• Implicated in lactic acidosis• Other possible manifestations:– Neuropathy (d4T, ddI) – Lipoatrophy (d4T)– Pancreatitis (ddI)– Myopathy (ZDV)– Cardiomyopathy (d4T, ZDV)

25

Facial Lipoatrophy: may be due mitochondrial toxicity.Not reversible, occur commonly with stavudine

Facial Lipoatrophy:

26

Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTI)

• Nevirapine (NVP)• Efavirenz (EFV)• Delavirdine (DLV)

• Etravirine

27

NNRTI Mechanism of Action

28

NNRTI Mechanism of Action cont…

• NNRTIs also inhibit the viral reverse transcriptase enzyme but have a different mechanism of action compared to NRTIs.

• NNRTIs bind directly to the reverse transcriptase enzyme.– Inhibit viral DNA synthesis.

• Unlike the NRTI agents, NNRTIs neither compete with nucleoside triphosphates nor require phosphorylation to be active.

• All are substrates for CYP3A4 and can act as inducers (nevirapine), inhibitors (delavirdine), or mixed inducers and inhibitors (efavirenz).

29

Efavirenz• Should be taken on an empty stomach because high-fat meal

increase toxicity• metabolized by CYP3A4 and CYP2B6 to inactive

metabolites• It is highly bound to albumin (~ 99%)• The principal adverse effects: dizziness, drowsiness,

insomnia, headache, confusion, amnesia, agitation, delusions, depression, nightmares, euphoria and Skin rash

• It is both inducer and an inhibitor of CYP3A4.• Contraindicated in pregnancy (1st trimester)!!– Known to cause birth defects

30

Nevirapine• oral bioavailability (90%) is not food-dependent• It is extensively metabolized by the CYP3A isoform• When initiating therapy, gradual dose escalation over 14 days

is recommended to decrease the incidence of rash• Hepatotoxicity occurs in about 4% of patients.• a moderate inducer of CYP3A metabolism.• Rash occurs in approximately 17% of patients

31

Nevirapine: Moderate Rash

32

Nevirapine: Severe Rash

33

Protease Inhibitors (PIs)• Lopinavir• Nelfinavir • Indinavir • Saquinavir • Ritonavir• Amprenavir• Atazanavir• Fosamprenavir• Tipranavir• Darunavir

34

Protease Inhibitors (PIs) cont…

• Protease inhibitors– prevent viral protease enzyme from cleaving the

polyprotein precursor to viral coat protein and reduces activation of critical viral proteins/enzymes.

• Thus new virons are formed, but are defective and cannot infect other cells.

• Use of PIs is associated with a syndrome of redistribution and accumulation of body fat that results in central obesity to the exception of atazanavir.

35

PI Mechanism of Action

36

PI Mechanism of Action• Protease enzyme– responsible for cleaving (cutting up) larger polyproteins

into structural proteins and reverse transcriptase enzyme.• Protease is needed to form a fully mature, functional virus

that is able to replicate and produce more virus• Protease inhibitors prevent this enzyme from doing its job in

the later steps of the viral life cycle

37

PI Class Side Effects• Metabolic Disorders– Hepatotoxicities– Hyperglycemia, insulin resistance– Lipid abnormalities– Fat redistribution

• Bone Disorders• GI intolerance• Drug interactions• CYP450 3A4 Inhibition– RTV > IDV = NFV = APV >SQV

38

Lopinavir/Ritonavir

• Combination in which subtherapeutic doses of ritonavir inhibit the CYP3A4-mediated metabolism of lopinavir.

• Ritonavir is acting as a pharmacokinetic enhancer rather than an antiretroviral agent.

• Absorption of lopinavir is enhanced with food • Lopinavir is extensively metabolized by the CYP3A

isozyme.• Toxicity– Nausea, – diarrhea– Lipid abnormalities– Hyperglycemia

39

Ritonavir• an inhibitor of HIV-1 and HIV-2 proteases • Metabolized by 3A4 > 2D6• Ritonavir potent inhibitor of liver enzymes, especially CYP450 3A4.

– CYP3A4 inhibitory properties exploited to prolong the half-life of more potent and less toxic PI agents.

• Ritonavir is rarely used as a single PI anymore b/c of its side effect profile.

• Instead, it is more often combined with other PIs to take advantage of its pharmacokinetic inhibitory properties.

• Toxicity– GI intolerance– Tingling around mouth, bitter taste in mouth– Hepatitis, use with caution in hepatic impairment– Lipodystrophy, lipid abnormalities

40

Fat redistribution

“Buffalo Hump” Central

Obesity

41

Fat redistribution

Lipoatrophy

42

Fusion Inhibitors• Include: Enfuvirtide• Inhibits entry of HIV into the CD4 cell. • binds to glycoprotein gp41 (a protein on the viral membrane).– prevents a change in the shape of the membrane protein and

prevents fusion of the virus and the CD4 cell membrane.– blocks entry into the cell

• A synthetic 36-amino-acid peptide• Unfortunately, Enfuvirtide is only active when injected

subcutaneously.• This aspect (in addition to its high cost) severely limits its use

in the correctional setting.• Enfuvirtide lacks cross-resistance to the other currently

approved antiretroviral drug classes.

43

Maraviroc• First in new class of agents, CCR5 inhibitors

• Approved August 6th, 2007

• Maraviroc binds to the CCR5 receptor on the membrane of human cells such as CD4 cells.

– prevents the interaction of HIV-1 gp120 and human CCR5 which is necessary for entry into the cell.

• Does not prevent HIV-1 entry into CXCR4-tropic or dual-tropic cells.

• Indicated (in combination with other ARVs) treatment-experienced adult HIV-infected patients

• Not recommended in patients who have dual/mixed tropic or CXCR4-tropic virus

• Use of maraviroc should be based on treatment history and tropism assay results.

44

Intergrase Inhibitors:

Raltegravir• A pyrimidinone analog that binds and inhibits the enzyme

integrase.– It inhibits strand transfer,

• Bioavailability does not appear to be food-dependent• Metabolized by glucuronidation.

45

Ethiopian ARV Guidelines (2008) - First-line Regimens

Recommended ARV Regimens for Adults and Adolescents:

Preferred : TDF+FTC/3TC+EFV = Triple FDC ZDV+3TC+EFV = ZDV/3TC + EFV ZDV+3TC+NVP = Triple FDC

Alternatives: D4T/3TC/EFV = Double FDC (d4T/3TC) + EFV TDF/3TC/NVP D4T/3TC/NVP = Triple FDC ABC/3TC/EFV ABC/3TC/NVP ABC/3TC/ZDV = ZDV/3TC + ABC

46

Second-line Regimens Recommended ARV Regimens for Adults and Adolescents:

First-line Regimen Second-line Regimen

TDF+FTC/3TC+EFV/NVP ZDV+3TC+LPV/r/ATV/r orZDV+ABC+LPV/r/ATV/r

d4T+3TC+EFV/NVP TDF+3TC+/-ZDV+LPV/r/ATV/r orABC+ ddI+ LPV/r/ATV/r

ABC + 3TC + ZDV EFV/NVP + LPV/r/ATV/r

47

PMTCT

ARVs decrease risk of MTCT by two mechanisms:

1.By reducing the viral load in the mother

ARVs block viral replication →↓Viral load in the body→↑CD4cells →Improved maternal immunity

2.By preventing the virus from fixing itself in the infant

48

Table: Three options for PMTCT programmes (WHO 2012)

a Recommended in WHO 2010 PMTCT guidelinesb True only for EFV-based first-line ART; NVP-based ART not recommended for prophylaxis (CD4 >350)c Formal recommendations for Option B+ have not been made, but presumably ART would start at

diagnosis.

49

Reading Assignments

• Chemotherapy of protozoal infections• Antifungals• Anthelmintics