Anti-HIV Chemotherapy

Go here for the current status of specific anti-HIV drugs and links to drug data sheets

Proteolysis of viral polyproteins (Host protease in the Golgi Body processes gp160, Viral protease processes GAG,POL)

Until recent years, few drugs have shown sufficient activity against HIV at non-toxic concentrations to warrant further development. Effective agents must show activity against virus in T4 cells, macrophages, neural cells, epithelial cells etc. and thereby eliminate virus from the patient.

Most agents are virustatic rather than virucidal. This is, perhaps, all we can expect but there are some hopes that a chemotherapeutic approach could lead to the eventual eradication of HIV from the bodies of at least some individuals. However, there are major doubts that eradication can ever be achieved because of the very slow clearance of HIV-infected resting memory T cells.

At present, the chemotherapeutic approach that is most likely to eradicate the virus is the highly active anti-retroviral therapy (HAART) or triple drug treatment. This consists of two nucleoside RT inhibitors plus a protease inhibitor or a nucleoside RT inhibitor, a non-nucleoside RT inhibitor and a protease inhibitor (see below).

These are competitive inhibitors of reverse transcriptase since they bind to the enzyme's active site.

In addition, they are DNA chain terminators, that is they are recognized by reverse transcriptase as nucleotides and incorporated into DNA; however, they usually lack the complete deoxyribose ring or a hydroxyl group at carbon 3 of the sugar and therefore their structure precludes the further addition of nucleotides since a phosphodiester bond cannot be formed. The first of these competitive reverse transcriptase inhibitors was azidothymidine (AZT). It has a far higher affinity for reverse transcriptase that it does for the host cell DNA polymerase but it also inhibits cellular enzymes leading to severe side effects.

Several reverse transcriptase inhibitors have been approved for use against HIV, none are without side-effects. These are:

Drug	Trade Names	Link
AZT (3'-Azidothymidine)	Zidoudine, Retrovir	ZIDOVUDINE - AZT, ZDV (Retrovir)
DDI (2',3'-dideoxyinosine)	Didanosine, Videx	DIDANOSINE - VIDEX
DDC (2',3'-didexoycytidine)	Zalcitabine, Hivid	ZALCITABINE (ddC, HIVID®)
d4T (Didehydrothymidine)	Stavudine	STAVUDINE
3TC (2'-deoxy-3'-thiacytidine)	Lamivudine, Epivir	FDA Approves Epivir For Use In HIV Patients Three-Months Old And Up
Abacavir succinate	Ziagen	Triple Therapy With Ziagen Effective For Up To 48 Weeks
Previon	Adefovir
For more information go here

AZT is the only nucleoside inhibitor that has been subjected to a placebo-controlled trial. When originally used as the first anti-HIV drug, it was administered when patient CD4 cells were <500 cells/mm³. It delays onset of AIDS-related opportunistic infections but in a controlled trial it was found that the use of AZT in asymptomatic HIV-infected patients resulted in no difference in survival at 3 years.

Before the results of a major trial published in April 1993, AZT monotherapy was considered: "Mediocre---but better than nothing"

After April 1993, the consensus was that AZT offers "no significant benefit to infected healthy people in slowing the disease or prolonging life" (Concorde trial France/UK. Lancet)

When non-syncytium-forming viruses convert to syncytium-forming, there is a change from clinical latency to a rapid decline. Unfortunately, AZT is minimally effective against syncytium-forming version of virus.

Nevertheless, AZT is extremely effective at preventing perinatal transmission of HIV. Since the advent of the AZT monotherapy treatment and similar interventions, the number of perinatally acquired AIDS cases in the United States has dropped dramatically to around 150 per year, while the number of HIV-infected children born each year averages 300. The risk of mother-to-infant transmission of HIV has been found to range from 25% to 35% in an untreated individual. AZT treatment of the mother and her infant can reduce transmission risk from 26% to 8% and AZT/3TC and triple therapy reduce that risk even further.

DDI is a potent inhibitor of reverse transcriptase but is not superior to AZT. It is approved for persons on AZT for > 4 months or who cannot tolerate AZT.

DDC is the most potent of the three original nucleoside inhibitors when used on infected cells in culture. It is approved for use with AZT in adults with <300 CD4 per mm³ who have significant deterioration.

Abacavir sulfate (a synthetic carbocyclic nucleoside) is much less toxic than the above drugs

Why do RT inhibitors not work completely? Initially they reduce viral burden 50-90% but there is a failure to keep adequate drug levels over long periods. Subsequently, resistance develops after 0.5-1 year and occurs more rapidly in patients that have been infected for longer time. This is because they have greater burden of more diverse forms of the virus. Resistance results from mutations in reverse transcriptase which quickly arise because of enormous proliferation rate of virus and the lack of a proof reading activity in host polymerase II

These drugs are non-competitive reverse transcriptase inhibitors, that is they bind elsewhere than the active site of the enzyme.

Because of the problems with AZT and the other nucleoside analogs in the treatment of HIV, interest grew in another approach to inhibiting the same enzyme, reverse transcriptase. Alternative drugs might be useful in combination therapy since there is a limit to the number of mutations that reverse transcriptase can bear without losing function. Clearly, mutations resistant to a non-nucleoside non-competitive inhibitor of reverse transcriptase would be at a different site in the enzyme from the mutation that makes the enzyme resistant to a competitive nucleoside analog.

These are the most potent and selective reverse transcriptase inhibitors that we have, working at nanomolar concentrations. Unlike nucleoside analogs, they have minimal toxicity in tests with cultured cells (anti-viral activity occurs at 10,000 to 100,000-fold lower concentration than the cytotoxic concentration) and they have been shown to work synergistically with nucleoside analogs such as AZT. Moreover, they work against nucleoside analog-resistant HIV. Thus, these drugs have high therapeutic index and also show good bioavailablity so that anti-viral concentrations are readily achievable.

Not surprisingly, since these drugs target reverse transcriptase, resistant mutants rapidly emerge, even after only a few passages in cultured cells. In the clinic, resistant mutants also arise rapidly. They are therefore of little use in monotherapy; however, although resistant virus strains are cross resistant to other non-nucleoside reverse transcriptase inhibitors, they are not to nucleoside analog inhibitors. There is also some evidence that the drugs may be able to overcome resistance at the high concentrations that seem to be achievable.

There is now a collection of such agents that are chemically distinct. The three drugs of this type approved in the U.S. are nevirapine, delavirdine and efavirenz

In monotherapy, nevirapine gives an initial fall in HIV but resistance sets in and virus titers rise again to a high level. High concentrations that are achieved in the bloodstream may be of some use. A pediatric suspension of nevirapine (Viramune) has been approved by the US Food and Drug Administration for the treatment of HIV-infected infants and children. Safety, tolerability and pharmacokinetic trials involving nevirapine were conducted in more than 360 infants and children. It is now approved for combination therapy in adults. (Information Box on Adverse Reactions)
Nevirapine Useful in Combination HIV Therapy

Trials in combination with AZT show delay in emergence of resistant virus but resistance is NOT prevented. Disappointing results mean that this drug has not been developed.

Delavirdine (Rescriptor) treatment leads to considerable increases in CD4⁺ cells in combination therapy (with AZT and 3TC). There have been promising results in patients with very low CD4⁺ cells that have prior treatment with AZT. In combination with AZT and 3TC, DLV may delay emergence of resistance to AZT. The drug is absorbed rapidly. This drug is used in combination with a nucleoside analog such as AZT and the protease inhibitors discussed below. Use Of Rescriptor With Indinavir At Lower Doses Decreases Viral Load

One of the more important treatment developments at the 12th World AIDS Conference was the report of new phase III data showing that efavirenz (brand name Sustiva, formerly known as DMP-266), used in combination with other treatment, may suppress viral load at least as well as the protease inhibitor Indinavir in the equivalent combination with nucleoside reverse transcriptase inhibitors. The most noteworthy result was a comparison of viral load reduction with efavirenz plus AZT plus 3TC, vs. a standard-of-care control group treated with indinavir plus AZT plus 3TC. The efavirenz combination suppressed viral load to below 400 copies in a significantly higher proportion of the volunteers than the control arm, at all time points between week 2 and week 24. This drug appears to cause major problems in pregnant women. It is teratogenic.

Since the introduction of AZT and similar analogs, new avenues of anti-HIV chemotherapy have been explored since these drugs are clearly not sufficient as anti-HIV agents. This is because of their side effects and, more importantly because of:

AZT has no impact on survival. Combinations of nucleoside inhibitors are more effective in that they reduce plasma HIV RNA to about 10% of the initial value and maintain this for up to 2 years. This can slow development of disease but not prevent it

As always, there remains the problem of mutation of RNA viruses, a phenomenon that results in the rapid evolution of drug-resistant viral quasi species. In an HIV-infected person, about 10.3 x 10⁹ virions are produced per day. These have a half life of 5.7 hours and a fixed mutation rate of 3 x 10^-5 per base per replication cycle in an HIV genome of 10⁴ base pairs. This means that at least one mutation may occur in each nucleotide of HIV in a day.

After a year from seroconversion, each infected individual establishes his/her own set point of steady state levels of plasma HIV RNA (10² to 10⁷ copies per ml of blood). This level largely determines the rate at which T4 cells are lost. It appears that this level of viral load drives the rate of immune destruction and can predict the course of the disease. Reduction in viral load appears to account for most of the clinical benefits of any drug. Clearly, if viral load is reduced to negligible levels, we have a very important drug. If the negligible levels are achieved by suppression of replication, the evolution of resistant mutants may not be a problem since mutation requires replication.

Recently there has been renewed optimism that HIV might become a tractable disease since now we have the promise of a drug regimen that can suppress indefinitely the progress of disease. If, as seems to be the case, viral load is the key to the progression from infection to the clinical manifestations of the disease, this combination therapy may make HIV infection manageable.

Many aspartyl protease inhibitors are being developed but at present, only a few (five by October 2000) are approved. They are all substrate analogs, that is they mimic a peptide that can bind to the active site of the viral protease. The latter is a dimeric aspartyl protease (has catalytic aspartates at its active site). When used individually, protease inhibitors can drive down viral load to between one 30^th and one 100^th of initial value but sub-optimal doses of these inhibitors when used alone can result in loss of suppression after several weeks or months and an accumulation of multiple mutations in the protease gene giving a high level of drug resistance. Note: patients with sustained suppression do not develop the resistant mutations. This is because replication must be maintained for the development of such mutations under the selective pressure of the drug.

Saquinavir (SQ) (Hoffman-La Roche). This is a hydroxyethylamine transition-state analog of the HIV protease cleavage site. It is the least bio-available of the present protease inhibitors and is the least effective. Nevertheless, SQ + AZT + ddC reduced viremia with a rise in T4 cells in individuals with a T4 cell count of 50 - 300/mm³. SQ plus ddC versus any drug alone in individuals with prior AZT treatment showed significant benefit.

Ritonavir (Abbot Labs). Reduces AIDS-defining events and death by 58% compared to placebo. Causes nausea in 25% of patients (Abbot Labs).

Indinavir (Merke). Indinavir plus two anti-reverse transcriptase drugs (AZT and 3TC) reduces HIV to such an extent that PCR cannot detect the virus in 85% of patients. Initial levels in the patients tested were 20,000 to 11,000,000 RNA copies per cu mm of blood (The threshold for detection by PCR is now in the area of 20 copies per ml) This suppression has lasted up to 1 year in more than 90% of patients.

Decline in the number of HIV particles in patients treated with a triple drug regimen (HAART). Clinically infected patients were treated with HAART. Plasma viremia declines in two phases: Within 2 weeks, it has declined by 99%. This is followed by a slower decline; by 8 weeks, HIV viral titers have declined below the limits of detection of the assay

The levels in the treated patients are below those in the blood of long term non-progressors who have remained clinically stable and shown no loss of T4 cells for over a decade. It was hoped that HAART treatment might lead to the possibility of purging the patient of the virus as infected cells turned over in the apparent absence of reinfection by released virus particles. It is now known that latent, persistently infected CD4+ cells remain in the body for long periods (Information Box)

Note: It was formerly the practice to add one additional drug to the treatment as the patient deteriorated. However, this will result in selection of resistant virus and preclude the benefits of combination therapy. If eradication becomes possible, aggressive early treatment is indicated.

Note: The use of the HAART regimen of drugs can lead to undetectable levels of HIV in the bloodstream but there is still HIV in genital secretions. Thus these persons are still infective. Recently, patients have been appearing who have clearly been infected by a person who harbors protease inhibitor-resistant virus. This means that patients on the HAART regimen are again having unprotected sex with their partners Drug-Resistant Strains Of HIV Can Be Passed On

At present these drugs must be taken three times a day, every day.. forever, unless they prove to eradicate the virus from the patient. Currently protease inhibitors cost about $7000 per year. The cost of zidovudine/lamivudine/protease inhibitor regimen is $12,000 per year. These drugs are, therefore, no answer to the worldwide AIDS epidemic.

Drug	Brand Name
Saquinavir	Invirase, Fortonase
Ritonavir	Norvir
Indinavir	Crixivan
Nelfinavir	Viracept
For more information go here

Although HAART combination therapy has proved highly effective in suppressing viral loads in infected patients, other avenues of treatment are being explored since HAART is not effective in all patients and because of the high costs of this drug combination.

It would be clearly logical to target the receptors for HIV in the cell surface since this is the initial stage of infection and is also the rate-limiting step in infection. The first stage in entry is adhesion probably to a charged molecule such as heparan sulfate proteoglycan but it should be noted that the actual mechanism of initial attachment may depend on the cell type. Little progress has been made here.

The uptake of the virus involves the CD4 antigen, the receptor, and chemokine receptor co-receptors. Blocking the interaction of HIV gp120 with either of these could potentially inhibit infection. Agents that bind to either receptor or gp120 might be useful but none of the agents below can be taken orally.

Soluble CD4
A preparation of the region of the CD4 molecule that binds to gp120, would clearly block attachment to CD4 on the cell surface and soluble CD4 blocks infection in culture. Unfortunately, it has little effect clinically. It may not be practical since the protein gets broken down rapidly. In addition, it can exacerbate infectivity as it promotes binding of HIV to chemokine receptors.

A more stable version of soluble CD4 is a tetrameric fusion protein of immunoglobulin G and CD4. It can reduce levels of virus in vivo.

CD4-PEG (Pseudomonas aeruginosa endotoxin A)
This drug synergises with AZT but may have too great a non-specific toxicity

The involvement of co-receptors was discovered when some chemokines were shown to block infection. CCR5 is a co-receptor for HIV and CCR5 deletions make the infected patients resistant to HIV. Persons with this deletion appear normal and healthy. This suggested that targeting CCR5 might block HIV in patients without any severe effect on the immune system. RANTES is a chemokine that binds to CCR5 and a RANTES analog (n-nonanoyl (NNY)-RANTES) has been shown to block HIV infection without triggering any measured intracellular signal from the receptor. However, although CCR5 co-receptor antagonists are effective in inhibiting non-syncytium-inducing HIV replication, they provide a selective pressure for the outgrowth of syncytium-inducing (X4) virus strains which are resistant to the RANTES analog and can use the CXCR4 co-receptor. Morever, However, chemokine derivatives are proteins (or at least peptides) and so will no be orally available and will probably have short half lives in vivo. Monoclonal antibodies against the co-receptor might be better and work well to block infection in vitro but they will also not be available orally.

Small molecule co-receptor antagonists have been found. These are often highly negatively charged molecules. In addition cationic peptides derived from the V3 sequence of gp120 seem to work. Because of their charge, small molecule antagonists of the co-receptors have very low oral bioavailability but AMD3100 is undergoing clinical trials in which it is administered by injection. It appears to bind to CXCR4 (fusin) and block the interaction between CXCR4 and the V3 loop.

Peptides derived from gp41 can inhibit infection, probably by blocking the interaction of gp41 with cell membrane proteins during fusion or by stopping the conformational change that results from the association of two gp41 molecules and which is necessary for fusion. T-20 block this conformational change. In clinical trials, a nearly two log reduction in plasma viral levels was achieved.

There is a cavity on gp41 that could hold a small molecule inhibitor. Peptides containing D-amino acids that would fit this cavity have been identified and inhibit fusion

Antibodies that bind to CD4 antigen are likely to block virus attachment but may be immunosuppressive because they will lead to depletion of CD4 cells.

The process of uncoating is not well understood but appears to involve a cell protein called cyclophilin A which appears to be needed for a productive infection by HIV. This cell protein is actually packaged in the viral capsid. However, in vitro studies show that mutants can evolve that no longer require cyclophiilin A.

The matrix protein is myristoylated and the surface glycoprotein is glycosylated. The addition of these post-translational modifications offers a potential target for drug therapy.

Blockers of myristoylation
Analogs of myristic acid that block myristoyl coA:Myristoyl transferase such as 4-oxatetradecanoic acid are non-toxic and work well in culture. Clinical efficacy unknown.

Castanospermine, a naturally occurring alkaloid and inhibitor of glucosidase-I has some effect against HIV in vitro. This has led to the development of analogs such as N-Butyl deoxynojirimycin(butyl-DNJ) and 6-butyl-castanospermine which have more potent activity against HIV

Several possibilities are being investigated. These include:
i) Anti-sense RNAs that bind to HIV mRNAS and block their translation
(ii) catalytic RNAs (ribozymes)
(iii) Oligonucleotides that target the nucleic acid-binding control proteins of HIV such as TAT and REV.

Thiol-based reagents
N-acetyl cysteine and procysteine can inactivate HIV in latently infected cells where they raise intracellular GSH. 1,2-dithiole-3-thiones raise GSH and also irreversibly inhibit reverse transcriptase .

Recent links to other new chemotherapeutic approaches to treating HIV infection

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