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
– Has killed over 22 million people.
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.
How HIV Enters Cells
• There are three crucial steps for entry of HIV into the CD4 T
1.Binding of HIV via the gp120 envelope protein to the CD4
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.
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.
• 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
• HIV can lie dormant within a cell for many years,
especially in resting (memory) CD4 cells, unlike other
• 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.
HIV Life Cycle and Sites for Therapeutic Intervention
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
• Nucleoside reverse transcriptase inhibitors (NRTIs) or nukes
– Mimic naturally occurring nucleosides
– Blocks viral DNA construction as they deceive reverse
• 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.
NRTI Mechanism of Action
• NRTIs must first undergo intracellular phosphorylation to
• 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
• They prevent the addition of the natural nucleosides into the
• This halts the production of new virions
NRTI Class Side Effects
Peripheral Neuropathy (DDC > d4T/ddI)
Pancreatitis (ddI > d4T)
– d4T > ddI > ZDV
– Rare with ABC, TDF, 3TC and FTC
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.
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
• Combinations containing AZT plus d4T, is therefore
pointless, since both drugs would compete for the same
• The combination of these drugs is antagonistic in vitro and in
vivo: do not use together.
• A synthetic nucleoside (cytidine) analogue reverse
• Active against HIV-1, HIV-2, and hepatitis B virus.
• Phosphorylated 3 times by thymidine kinase to the active
• Competes with deoxycytidine triphosphate (dCTP) for
binding to reverse transcriptase and incorporation of the
nucleoside analogue into viral DNA results in the
• 3TC-TP is a weak inhibitor of mammalian DNA polymerases
α, β, and γ.
• Bioavailability (F): 86%.
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
– 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.
Lamivudine (3TC) cont…
• Intracellular T1/2
– NRTIs work in the cell, duration of action is based on
• Toxicity: very rare
– Occasional nausea
– Lactic acidosis
Tenofovir Disoproxil Fumarate (TDF)
• TDF is different from nucleosides in that it has already been
• 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.
– Headache, Nausea, diarrhea
– Lactic acidosis, fatty liver
– Renal insufficiency (rare)
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)
Facial Lipoatrophy: may be due mitochondrial toxicity.
Not reversible, occur commonly with stavudine
NNRTI Mechanism of Action cont…
• NNRTIs also inhibit the viral reverse transcriptase enzyme
but have a different mechanism of action compared to
• 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
• All are substrates for CYP3A4 and can act as inducers
(nevirapine), inhibitors (delavirdine), or mixed inducers and
• Should be taken on an empty stomach because high-fat meal
• metabolized by CYP3A4 and CYP2B6 to inactive
• 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
• 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
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.
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
PI Class Side Effects
• Metabolic Disorders
– Hyperglycemia, insulin resistance
– Lipid abnormalities
– Fat redistribution
• Bone Disorders
• GI intolerance
• Drug interactions
• CYP450 3A4 Inhibition
– RTV > IDV = NFV = APV >SQV
• 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.
– Lipid abnormalities
• 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
• Instead, it is more often combined with other PIs to take advantage of
its pharmacokinetic inhibitory properties.
– GI intolerance
– Tingling around mouth, bitter taste in mouth
– Hepatitis, use with caution in hepatic impairment
– Lipodystrophy, lipid abnormalities
• 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
– blocks entry into the cell
• A synthetic 36-amino-acid peptide
• Unfortunately, Enfuvirtide is only active when injected
• 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.
• 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
• Indicated (in combination with other ARVs) treatment-experienced
adult HIV-infected patients
• Not recommended in patients who have dual/mixed tropic or
• Use of maraviroc should be based on treatment history and
tropism assay results.
• A pyrimidinone analog that binds and inhibits the enzyme
– It inhibits strand transfer,
• Bioavailability does not appear to be food-dependent
• Metabolized by glucuronidation.
Second-line Regimens Recommended ARV Regimens
for Adults and Adolescents:
ABC+ ddI+ LPV/r/ATV/r
ABC + 3TC + ZDV
EFV/NVP + LPV/r/ATV/r
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
Table: Three options for PMTCT programmes (WHO 2012)
Recommended in WHO 2010 PMTCT guidelines
b 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