Anti-HIV DrugsAnti-HIV Drugs

Melissa MorganMelissa Morgan

Medicinal ChemistryMedicinal Chemistry

November 23, 2004November 23, 2004

The Life Cycle of HIVThe Life Cycle of HIV HIV causes the depletion of CD4 HIV causes the depletion of CD4

T-cells in the immune systemT-cells in the immune system A CD4 receptor protein and a co-A CD4 receptor protein and a co-

receptor, such as CCR5 or CXCR4, receptor, such as CCR5 or CXCR4, are required for HIV to enter a cellare required for HIV to enter a cell

Fusion of the viral envelope with Fusion of the viral envelope with the cell membrane allows the the cell membrane allows the viral genome and proteins to viral genome and proteins to enter cellenter cell

Reverse-transcriptase produces a Reverse-transcriptase produces a cDNA copy of the viral RNAcDNA copy of the viral RNA

Viral integrase incorporates viral Viral integrase incorporates viral cDNA into host DNA as proviruscDNA into host DNA as provirus

Transcription and translation of Transcription and translation of viral proteins occurviral proteins occur

Capsids assemble around viral Capsids assemble around viral genomes and enzymesgenomes and enzymes

New virus particles bud from host New virus particles bud from host T-cell after assemblyT-cell after assembly

Infected T-cell eventually dies Infected T-cell eventually dies

The Reverse-Transcriptase The Reverse-Transcriptase EnzymeEnzyme

The primary source of The primary source of scientific results on scientific results on reverse-transcriptase is reverse-transcriptase is conducting X-ray conducting X-ray crystallography studies on crystallography studies on the enzyme the enzyme

The catalytic p66 subunit The catalytic p66 subunit of reverse-transcriptase of reverse-transcriptase has 4 domains, shown here has 4 domains, shown here as different colored regions as different colored regions of the ribbon diagramof the ribbon diagram

Several classes of anti-HIV Several classes of anti-HIV drugs target the actions of drugs target the actions of reverse-transcriptase reverse-transcriptase Reverse-transcriptase active site

The Classes of Anti-HIV The Classes of Anti-HIV DrugsDrugs

Nucleoside reverse-transcriptase Nucleoside reverse-transcriptase inhibitors (NRTIs)inhibitors (NRTIs)

Nucleotide reverse-transcriptase Nucleotide reverse-transcriptase inhibitorsinhibitors

Non-nucleoside reverse-transcriptase Non-nucleoside reverse-transcriptase inhibitors (NNRTIs)inhibitors (NNRTIs)

HIV Protease InhibitorsHIV Protease Inhibitors Entry Inhibitors – includes the chemokine Entry Inhibitors – includes the chemokine

receptor binders and the gp41-receptor binders and the gp41-dependent membrane fusion inhibitors dependent membrane fusion inhibitors

Sites of Action of Anti-HIV Sites of Action of Anti-HIV DrugsDrugs

Nucleoside Reverse-Nucleoside Reverse-Transcriptase Inhibitors (NRTIs)Transcriptase Inhibitors (NRTIs) NRTIs must be phosphorylated 3 times by kinases to NRTIs must be phosphorylated 3 times by kinases to

form nucleoside triphosphates once inside the cellform nucleoside triphosphates once inside the cell This causes reverse-transcriptase to incorporate the This causes reverse-transcriptase to incorporate the

drug, rather than the natural nucleoside drug, rather than the natural nucleoside triphosphate, thus terminating the growth of the triphosphate, thus terminating the growth of the DNA strandDNA strand

Drugs in this class include zidovudine, didanosine, Drugs in this class include zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, and zalcitabine, stavudine, lamivudine, abacavir, and emtricitabine emtricitabine

2 mechanisms associated with HIV resistance to the 2 mechanisms associated with HIV resistance to the NRTIs: NRTIs:

1.1. impairment of the enzyme’s ability to incorporate an impairment of the enzyme’s ability to incorporate an analog into DNA analog into DNA

2.2. removal of the analog from the prematurely removal of the analog from the prematurely terminated DNA strand terminated DNA strand

Zidovudine (AZT)Zidovudine (AZT) AZT was originally developed in 1964 as a potential anti-cancer agent,

but was found to be ineffective In the mid-1980s, AZT was found to be effective in fighting HIV as a

result of a screening process aimed at identifying anti-HIV agents AZT works because it is an analog of thymidine that can be incorporated

into the DNA strand Normally, the 3’ –OH group of thymidine binds to the phosphate group of Normally, the 3’ –OH group of thymidine binds to the phosphate group of

the next nucleotide in the DNA strand the next nucleotide in the DNA strand However, AZT has an azido group instead of an –OH group, and the azido However, AZT has an azido group instead of an –OH group, and the azido

group cannot bind to a phosphate group group cannot bind to a phosphate group As a result, reverse transcription stops once AZT is incorporated into the As a result, reverse transcription stops once AZT is incorporated into the

DNA strand and incomplete proviral DNA is produced DNA strand and incomplete proviral DNA is produced

Nucleotide Reverse-Nucleotide Reverse-Transcriptase InhibitorsTranscriptase Inhibitors

Same mechanism of action as NRTIs, but only 2 phosphorylation Same mechanism of action as NRTIs, but only 2 phosphorylation events are required to convert drug to its active triphosphate events are required to convert drug to its active triphosphate formform

These drugs compete with normal DNA substrates to act as These drugs compete with normal DNA substrates to act as chain termination inhibitors of reverse-transcriptasechain termination inhibitors of reverse-transcriptase

The mutations that confer resistance to the NRTIs also confer The mutations that confer resistance to the NRTIs also confer resistance to the nucleotide reverse-transcriptase inhibitors resistance to the nucleotide reverse-transcriptase inhibitors

The only FDA-approved drug in this class is tenofovir disoproxil The only FDA-approved drug in this class is tenofovir disoproxil fumarate, a prodrug that is converted to its active form, fumarate, a prodrug that is converted to its active form, tenofovir, once in the bodytenofovir, once in the body

Structure of tenofovir disoproxil fumarate: Structure of tenofovir disoproxil fumarate:

Non-Nucleoside Reverse-Non-Nucleoside Reverse-Transcriptase Inhibitors Transcriptase Inhibitors

(NNRTIs)(NNRTIs) These drugs work by binding to an allosteric site, These drugs work by binding to an allosteric site,

specifically a hydrophobic pocket located near the specifically a hydrophobic pocket located near the catalytic domain of reverse-transcriptasecatalytic domain of reverse-transcriptase

The binding of the inhibitor restricts the activity and The binding of the inhibitor restricts the activity and mobility of the enzyme, thus blocking the polymerization mobility of the enzyme, thus blocking the polymerization of viral DNAof viral DNA

These drugs do not have to be activated by kinases to These drugs do not have to be activated by kinases to form phosphate esters like the NRTIsform phosphate esters like the NRTIs

Increasing the amount of substrate does not displace the Increasing the amount of substrate does not displace the drug from the enzyme; therefore, NNRTIs demonstrate drug from the enzyme; therefore, NNRTIs demonstrate non-competitive inhibitory action with the enzymenon-competitive inhibitory action with the enzyme

Drugs in this class include nevirapine, delavirdine, and Drugs in this class include nevirapine, delavirdine, and efavirenzefavirenz

Mutations that confer resistance to NNRTIs are located in Mutations that confer resistance to NNRTIs are located in the hydrophobic pocket targeted by the drugs, and they the hydrophobic pocket targeted by the drugs, and they act by reducing the affinity of the drug for the site act by reducing the affinity of the drug for the site

Examples of NNRTIsExamples of NNRTIs

Nevirapine Efavirenz

Delavirdine

HIV Protease InhibitorsHIV Protease Inhibitors The 3-dimensional structure of the The 3-dimensional structure of the

HIV-1 protease was determined in HIV-1 protease was determined in 1988 when the enzyme was 1988 when the enzyme was crystallizedcrystallized

The structure consists of a dimer The structure consists of a dimer demonstrating precise dual demonstrating precise dual rotational C2 symmetryrotational C2 symmetry

The active sites are located in The active sites are located in loops that approach the center of loops that approach the center of the dimerthe dimer

HIV-1 protease develops the gag HIV-1 protease develops the gag and gag-pol polyproteins into and gag-pol polyproteins into functional viral proteins & enzymesfunctional viral proteins & enzymes

The structures of the HIV protease The structures of the HIV protease inhibitors are derived from the inhibitors are derived from the natural peptidic substrates of the natural peptidic substrates of the HIV-1 protease HIV-1 protease

These drugs work by binding the These drugs work by binding the active site of HIV-1 protease, active site of HIV-1 protease, thereby preventing the enzyme thereby preventing the enzyme from releasing individual viral from releasing individual viral proteinsproteins

HIV-1 protease with bound inhibitor

Examples of HIV Examples of HIV Protease InhibitorsProtease Inhibitors

Ritonavir

Saquinavir Indinavir

Amprenavir

HIV Entry InhibitorsHIV Entry Inhibitors The HIV entry inhibitors class includes the chemokine The HIV entry inhibitors class includes the chemokine

receptor binders and the gp41-dependent membrane receptor binders and the gp41-dependent membrane fusion inhibitorsfusion inhibitors

The HIV-1 envelope glycoprotein contains 2 non-The HIV-1 envelope glycoprotein contains 2 non-covalently coupled subunits, gp120 and gp41covalently coupled subunits, gp120 and gp41

gp120 controls target cell recognition and viral gp120 controls target cell recognition and viral tropism through interaction with a CD4 receptor and a tropism through interaction with a CD4 receptor and a co-receptor, such as CCR5 or CXCR4, on the target cellco-receptor, such as CCR5 or CXCR4, on the target cell

Co-receptors are members of the seven-Co-receptors are members of the seven-transmembrane-spanning, G-protein-coupled receptor transmembrane-spanning, G-protein-coupled receptor family, whose normal function is to bind chemokinesfamily, whose normal function is to bind chemokines

Thus, gp120 is the subunit involved in the mechanism Thus, gp120 is the subunit involved in the mechanism of action of the chemokine receptor binding drugs of action of the chemokine receptor binding drugs

gp41 promotes fusion of the viral and cellular gp41 promotes fusion of the viral and cellular membranes membranes

HIV Entry InhibitorsHIV Entry Inhibitors

Chemokine Receptor Chemokine Receptor BindersBinders

Different strains of HIV-1 Different strains of HIV-1 employ different co-receptors employ different co-receptors for entryfor entry

The R5 strain utilizes the CCR5 The R5 strain utilizes the CCR5 co-receptorco-receptor

The X4 strain utilizes the The X4 strain utilizes the CXCR4 co-receptorCXCR4 co-receptor

The R5X4 strain utilizes both The R5X4 strain utilizes both CCR5 and CXCR4 co-receptorsCCR5 and CXCR4 co-receptors

AMD-3100 hinders only the X4 AMD-3100 hinders only the X4 strains of HIV-1 because it acts strains of HIV-1 because it acts as a selective inhibitor of the as a selective inhibitor of the CXCR4 co-receptor CXCR4 co-receptor

TAK-779 blocks only the R5 TAK-779 blocks only the R5 strains of HIV-1 because it strains of HIV-1 because it demonstrates selective binding demonstrates selective binding to the CCR5 co-receptorto the CCR5 co-receptor

Both of these drugs are Both of these drugs are currently in clinical trials currently in clinical trials

gp41-Dependent Membrane gp41-Dependent Membrane Fusion InhibitorsFusion Inhibitors

Fusion of the viral and cellular membranes elicits several Fusion of the viral and cellular membranes elicits several conformational changes that lead to the formation of the conformational changes that lead to the formation of the trimer-of-hairpins structure in gp41trimer-of-hairpins structure in gp41

The drugs in this class prevent membrane fusion by interfering The drugs in this class prevent membrane fusion by interfering with the development of the trimer-of-hairpins structure with the development of the trimer-of-hairpins structure

It is believed that a mechanism of dominant-negative It is believed that a mechanism of dominant-negative inhibition, involving C-peptides, prevents this development inhibition, involving C-peptides, prevents this development

One drug in this sub-class is enfuvirtideOne drug in this sub-class is enfuvirtide

Enfuvirtide (Fuzeon)Enfuvirtide (Fuzeon) Enfuvirtide is the only FDA-approved drug in the sub-Enfuvirtide is the only FDA-approved drug in the sub-

class of gp41-dependent membrane fusion inhibitorsclass of gp41-dependent membrane fusion inhibitors Enfuvirtide is a 36-amino acid peptide derived from Enfuvirtide is a 36-amino acid peptide derived from

the HR2 region of gp41 the HR2 region of gp41 During membrane fusion, HR2, a distal hydrophobic During membrane fusion, HR2, a distal hydrophobic

region of gp41, folds onto HR1, a proximal region of gp41, folds onto HR1, a proximal hydrophobic region, in order to shorten the molecule. hydrophobic region, in order to shorten the molecule.

Enfuvirtide binds to HR1 and inhibits the formation of Enfuvirtide binds to HR1 and inhibits the formation of the gp41 conformation necessary for fusion by the gp41 conformation necessary for fusion by interfering with the interaction between the COOH- interfering with the interaction between the COOH- and NHand NH22-terminal repeat -terminal repeat

Resistance to enfuvirtide arises as a result of Resistance to enfuvirtide arises as a result of mutations in a 10-amino acid motif in the HR1 region mutations in a 10-amino acid motif in the HR1 region of gp41 of gp41

Combination Therapy - Combination Therapy - HAARTHAART

Combinations of antiretroviral Combinations of antiretroviral drugs are also used for the drugs are also used for the treatment of HIV treatment of HIV

Highly active antiretroviral Highly active antiretroviral therapy, or HAART, enables therapy, or HAART, enables the pairing of different types the pairing of different types of drugs that may control or of drugs that may control or prevent the emergence of prevent the emergence of drug-resistant HIV strains drug-resistant HIV strains

HAART regimens typically HAART regimens typically include 3 drugs, usually 2 include 3 drugs, usually 2 nucleoside reverse-nucleoside reverse-transcriptase inhibitors and transcriptase inhibitors and either a protease inhibitor or a either a protease inhibitor or a non-nucleoside reverse-non-nucleoside reverse-transcriptase inhibitor transcriptase inhibitor

Future Anti-HIV Drug Future Anti-HIV Drug TargetsTargets

One step of the HIV life cycle that may be a future One step of the HIV life cycle that may be a future drug target is the RNase H activity of reverse-drug target is the RNase H activity of reverse-transcriptase. Screenings of compounds may enable transcriptase. Screenings of compounds may enable researchers to discover a drug that can attack that researchers to discover a drug that can attack that target. target.

Another potential drug target relates to the HIV virion Another potential drug target relates to the HIV virion infectivity factor (VIF). It is believed that the VIF infectivity factor (VIF). It is believed that the VIF prevents the activity of a cellular factor that normally prevents the activity of a cellular factor that normally stops the creation of infectious virions. stops the creation of infectious virions.

A final stage of the HIV life cycle that may be A final stage of the HIV life cycle that may be targeted by future drugs is the import of nucleic targeted by future drugs is the import of nucleic acids. Nuclear uptake is facilitated by virion proteins acids. Nuclear uptake is facilitated by virion proteins that may function with importin B and other nuclear that may function with importin B and other nuclear import receptorsimport receptors

These potential drug targets will undoubtedly be These potential drug targets will undoubtedly be exploited in the coming years exploited in the coming years

Summary of Anti-HIV DrugsSummary of Anti-HIV Drugs

ConclusionConclusion Current drugs provide Current drugs provide

many options to patients, many options to patients, through both monotherapy through both monotherapy and HAARTand HAART

HAART produces HAART produces particularly promising particularly promising clinical resultsclinical results

Future HIV drug treatment Future HIV drug treatment options must continue to options must continue to focus on the development focus on the development of drugs that are less likely of drugs that are less likely to encourage mutations to encourage mutations that confer resistancethat confer resistance

Hopefully, more successful Hopefully, more successful drugs to fight HIV and a drugs to fight HIV and a vaccine will be among the vaccine will be among the developments in the future developments in the future of HIV research of HIV research