Selection and characterization of human immunodeficiency virus type 1 variants resistant to the (+) and (-) enantiomers of 2'-deoxy-3'-oxa-4'-thio-5-fluorocytidine

Mechanisms of resistance to nucleoside analogue inhibitors of HIV-1 reverse transcriptase

Human immunodeficiency virus (HIV) reverse transcriptase (RT) inhibitors can be classified into nucleoside and nonnucleoside RT inhibitors. Nucleoside RT inhibitors are converted to active triphosphate analogues and incorporated into the DNA in RT-catalyzed reactions. They act as chain terminators blocking DNA synthesis, since they lack the 3'-OH group required for the phosphodiester bond formation. Unfortunately, available therapies do not completely suppress viral replication, and the emergence of drug-resistant HIV variants is facilitated by the high adaptation capacity of the virus. Mutations in the RT-coding region selected during treatment with nucleoside analogues confer resistance through different mechanisms: (i) altering discrimination between nucleoside RT inhibitors and natural substrates (dNTPs) (e.g. Q151M, M184V, etc.), or (ii) increasing the RT's phosphorolytic activity (e.g. M41L, T215Y and other thymidine analogue resistance mutations), which in the presence of a pyrophosphate donor (usually ATP) allow the removal of chain-terminating inhibitors from the 3' end of the primer. Both mechanisms are implicated in multi-drug resistance. The excision reaction can be modulated by mutations conferring resistance to nucleoside or nonnucleoside RT inhibitors, and by amino acid substitutions that interfere with the proper binding of the template-primer, including mutations that affect RNase H activity. New developments in the field should contribute towards improving the efficacy of current therapies.

Critical design features of phenyl carboxylate-containing polymer microbicides

Recent studies of cellulose-based polymers substituted with carboxylic acids like cellulose acetate phthalate (CAP) have demonstrated the utility of using carboxylic acid groups instead of the more common sulfate or sulfonate moieties. However, the pK(a) of the free carboxylic acid group is very important and needs careful selection. In a polymer like CAP the pK(a) is approximately 5.28. This means that under the low pH conditions found in the vaginal lumen, CAP would be only minimally soluble and the carboxylic acid would not be fully dissociated. These issues can be overcome by substitution of the cellulose backbone with a moiety whose free carboxylic acid group(s) has a lower pK(a). Hydroxypropyl methylcellulose trimellitate (HPMCT) is structurally similar to CAP; however, its free carboxylic acids have pK(a)s of 3.84 and 5.2. HPMCT, therefore, remains soluble and molecularly dispersed at a much lower pH than CAP. In this study, we measured the difference in solubility and dissociation between CAP and HPMCT and the effect these parameters might have on antiviral efficacy. Further experiments revealed that the degree of acid substitution of the cellulose backbone can significantly impact the overall efficacy of the polymer, thereby demonstrating the need to optimize any prospective polymer microbicide with respect to pH considerations and the degree of acid substitution. In addition, we have found HPMCT to be a potent inhibitor of CXCR4, CCR5, and dual tropic strains of human immunodeficiency virus in peripheral blood mononuclear cells. Therefore, the data presented herein strongly support further evaluation of an optimized HPMCT variant as a candidate microbicide.

Emerging reverse transcriptase inhibitors for the treatment of HIV infection in adults

A combination of three or more antiretroviral drugs, commonly called 'highly active antiretroviral therapy' (HAART), has become the standard-of-care treatment for HIV-infected patients in the developed world. There are now 21 licensed anti-HIV drugs to choose from when starting a HAART regimen. The currently approved antiretroviral drugs fall into four categories: nucleoside/nucleotide reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, protease inhibitors and fusion inhibitors. Novel compounds currently in preclinical or clinical development are either focusing on new viral proteins or the same specific viral elements targeted by the available drugs. When developing new anti-HIV drugs of an already existing class, focus should be held on maximising potency, minimising toxicity, diminishing the risk for resistance development and producing effective drugs for patients who already have resistance to currently available drugs. In addition, pill burden should be ideally reduced to once-daily dosing, thereby enhancing a patient's adherence and reducing treatment costs. The present review focuses on emerging drugs to inhibit the reverse transcriptase of HIV.

Resistance to New Anti-HIV Agents: Problems in the Pathway of Drug Registration

Resistance data are now requested by the regulatory agencies as an integral part of the approval process of new antiretroviral drugs. We examined the means by which resistance data was gathered during pre-clinical and clinical Phases I, II and III of drug development, and how the public and academic experts access these proprietary data. The analysis identified various opportunities for improvement of the current process, in particular the need for standards in generating and reporting resistance data on new antiretroviral drugs, and the need to enforce warnings in the product labelling on the drug combinations that can potentially lead to resistance and treatment failure.

Selection of resistance-conferring mutations in HIV-1 by the nucleoside reverse transcriptase inhibitors (+/-)dOTC and (+/-)dOTFC

The patterns of resistance-conferring mutations that are selected in HIV-1 reverse transcriptase (RT) by the racemates of 2'-dideoxy-3'-oxa-4'-thiocytidine (+/-)dOTC and its fluorinated derivative (+/-)dOTFC were characterized. Genotypic and phenotypic analyses of HIV-1 clinical isolates and HXB2D variants selected with (+/-)dOTC and (+/-)dOTFC were performed in primary cells and in the MT-2 T cell line. HIV-1 variants selected with (+/-)dOTC or (+/-)dOTFC displayed fivefold decreased susceptibility to the respective compounds. A substitution of methionine to valine was identified at position 184 (M184V) in variants selected with (+/-)dOTC. In contrast, a mutation of lysine to arginine at position 65 (K65R) was found in variants selected with (+/-)dOTFC. These patterns of selected mutations differ from those seen with the individual enantiomers. Studies with mutated recombinant HXB2D-M184V and -K65R confirmed that these mutations are important for phenotypic resistance in MT-2 cells. Clinical isolates that display resistance to (-)2'-deoxy-3'-thiacytidine (3TC) also showed cross-resistance to (+/-)dOTC and (+/-)dOTFC. These studies demonstrate that similar genotypes may be selected by the dOTC and dOTFC compounds to those with the structurally related drug 3TC.