Current Drug Metabolism - Volume 5, Issue 4, 2004

HIV therapy has well established and substantial clinical benefits. However, all antiretroviral agents can cause both short-term and long-term toxicities. This is a particularly vexing problem as current data does not allow accurate predictions of treatment toxicities and, in many cases, side effects are only partially reversible. Moreover, therapy is frequently associated with the selection of drug-resistant viral isolates and incomplete viral suppression. This is commonly caused by inadequate medication adherence and leads to increasingly severe virologic failure. The present issue will address both the advantages of current HIV therapy as well as the ongoing challenges in its application. A major focus will be on the development of novel strategies, both in terms of novel targets / drugs and of improved therapeutic applications of currently available medicaments.

One of the major advances in the recent history of the treatment of HIV infections has been the development of different classes of effective antiretroviral drugs. In particular, the reverse transcriptase (RT) inhibitors still represent the majority of the clinically used anti-HIV drugs and constitute the main backbone of currently employed combinatorial regimens. Highly active antiretroviral combination chemotherapy (HAART), combining RT and protease inhibitors, has proven the most effective approach to treat HIV disease, since it has been shown to markedly suppress viral replication and appearance of drug resistance for a relatively long period. These therapies, however, do not constitute a definitive cure, since they are not able to completely eradicate the virus from the infected individual. Beside drug toxicity problems, the emergence of drug resistance associated with the particular regimen employed further complicates the situation. This review will summarise the most recent achievements, as well as the future directions in the development of novel anti-RT compounds.

Human immunodeficiency virus (HIV) is the etiological agent of the acquired immune deficiency syndrome (AIDS). The current strategy for the treatment of HIV infection is called Highly Active Antiretroviral Therapy (HAART) and is based on cocktails of drugs that are currently approved by the Food and Drug Administration. These drugs include compounds that target the viral entry step and the enzymes reverse transcriptase or protease. The introduction of HAART has dramatically changed the landscape of HIV disease. Death from AIDS-related diseases has been reduced significantly since HAART came into use. Nevertheless it is not clear how long clinical benefit will last taking into account the emergence of multiple drug-resistant viral strains. Addition of new anti-HIV drugs targeting other steps of the viral replication cycle may increase the potency of inhibition and delay resistance development. HIV integrase is an essential enzyme in the HIV life cycle and is an attractive target for new drug development. Despite years of intensive research, only two classes of compounds that inhibit integration have been identified until now, namely the diketo acids and the pyranodipyrimidines. In this review we will point to new potential antiviral targets related to retroviral integration that are amenable to drug development. We will describe the pitfalls of currently used integrase assays and propose new strategies and technologies for the discovery of HIV integration inhibitors. Furthermore, we will describe the two classes of integrase inhibitors and discuss their antiviral activity, molecular mechanism of anti-HIV action and the selection of HIV resistance against these drugs.

The reverse transcriptase (RT) of human immunodeficiency virus type-1 (HIV-1) is an RNA- and DNAdependent DNA polymerase capable of copying the viral genome before it gets integrated into the human host DNA. Hence, HIV-1 RT plays a major role in viral replication and represents a key target for anti-AIDS treatments. Amongst the eleven licensed drugs that inhibit RT, eight are chain-terminating nucleoside analogues (NRTIs) that compete with their natural counterparts during the DNA polymerization process. Unfortunately, under therapeutic pressure, the HIV-1 inevitably develops resistance to these inhibitors by accumulating mutations in the viral pol gene encoding RT. Mechanisms for this resistance can be sorted in two categories, depending on the nature of the drug and the selected mutations. The first category includes mutations involving a specific alteration of the discrimination between natural nucleotides and NRTIs. The second category includes mutations able to promote the removal of the incorporated NRTI and thus repair the nascent DNA chain. This review summarizes the modes of inhibition of HIV-1 RT with NRTIs, and describes the mechanisms of resistance to these drugs, based on enzymatic data correlated to crystal structures and molecular models involving HIV-1 RT. We also give insights into different aspects of resistance such as antagonistic mutations, replication capacity, and the implications for a rational, structure-based drug design.

Resistance to antiretroviral drugs is one of the major pitfalls of combined treatment of HIV infection. Thus, timely identification of drug resistant HIV strains and precise evaluation of the level of resistance to the different antiretroviral drugs are crucial in the management of treated patients. Phenotypic determination of antiretroviral drug resistance evaluates the ability of an HIV strain to replicate in the presence of drug(s). Thus, this assay (either conventional or recombinant) provides a direct estimate of drug susceptibility. However, it is relatively difficult to perform and requires dedicated facilities. Thus, its use is still restricted. Genotypic determination of drug resistance is based upon detection of specific mutations in HIV genes encoding target enzymes or receptors. The assay provides an indirect evidence of drug susceptibility. However, it is easy to perform and does not require dedicated facilities. Thus, it is widely utilized in clinical practice. Its major limitation concerns the complexity of results interpretation which still awaits a general consensus.

HIV-1 protease is an aspartic protease composed by two identical monomers, 99 amino acids in length. Drug resistance is mainly mediated by structural changes in the substrate cleft that result in a reduction in drug binding affinity. Sequence analysis of drug resistance clones has shown mutations not only within the protease but also at several of the protease cleavage sites. Changes at more than 20 positions within the HIV-1 genome have been associated with PI resistance. The spectrum of mutations selected during therapy with indinavir, nelfinavir, saquinavir, ritonavir, amprenavir and atazanavir has been well characterized. Specific changes are characteristically linked to resistance to each of these compounds (i.e., D30N for nelfinavir, I50L for atazanavir or I50V for amprenavir). In contrast, for drugs such as lopinavir and tipranavir, which always are used boosted with low-dose ritonavir, combinations of multiple protease mutations rather than few specific changes seem to be necessary for causing significant drug resistance. Something similar happens when other PIs are equally boosted with ritonavir. Overall, when more than 5 protease resistance mutations are present, the response to any boosted-PI should be expected to be compromised.

A few α-L-fucosidase inhibitors and α-D-glucosidase inhibitors have shown in vitro anti-HIV activities, that have been attributed to their ability to inhibit HIV entry. The mechanism of action of inhibitors such as 1- deoxynojirimycin (1) is not clearly established. One possible hypothesis is that the glycosidase inhibition affects the final conformation of the glycoproteins involved in the virus / cell recognition and fusion phenomena. This hypothesis is presented critically and the mechanisms of some glycoprotein biosynthesis are out-lined. Up to now, very few glycosidase inhibitors have been assayed for their potential as HIV entry inhibitors. Further assaying should be done and larger collections of glycosidase inhibitors should be prepared. To help investigations in that perspective, the inhibitory activities of α-glucosidase and α-L-fucosidase inhibitors have been summarized.