Current Medicinal Chemistry - Volume 7, Issue 10, 2000

3'-Azido-2'3'-dideoxythymidine (AZT, 1, zidovudine, RetrovirTM) is used to treat patients with human immunodeficiency virus (HIV) infection. AZT, after conversion to AZT-5'-triphosphate (AZT-TP) by cellular enzymes, inhibits HIV-reverse transcriptase (HIV-RT). The major clinical limitations of AZT are due to clinical toxicities that include bone marrow suppression, hepatic abnormalities and myopathy, absolute dependence on host cell kinase-mediated activation which leads to low activity, limited brain uptake, a sort half-life of about one hour in plasma that dictates frequent administration to maintain therapeutic drug levels, low potential for metabolic activation and/or high susceptibility to catabolism, and the rapid development of resistance by HIV-1. These limitations have prompted the development of strategies for designing prodrugs of AZT. A variety of 5'-O-substituted prodrugs of AZT constitute the subject of this review. The drugdesign rationale on which these approaches are based is that the ester conjugate will be converted by hydrolysis and/or enzymatic cleavage to AZT or its 5'-monophosphate (AZT-MP). Most prodrug derivatives of AZT have been prepared by derivatization of AZT at its 5'-O position to provide two prominent classes of compounds that encompass: A) 5'-O-carboxylic esters derived from 1) cyclic 5'-O-carboxylic acids such as steroidal 17b-carboxylic acids, 1-adamantanecarboxylic acid, bicyclam carboxylic acid derivatives, O-acetylsalicylic acid, and carbohydrate derivatives, 2) amino acids, 3) 1,4-dihydro-1-methyl-3-pyridinylcarboxylic acid, 4)such as stearic acid, and 5) long chain polyunsaturated fatty acid analogs such as retinoic acid, and B) masked phosphates such as 1) phosphodiesters that include monoalkyl or monoaryl phosphate, carbohydrate, ether lipid, ester lipid, and foscarnet derivatives, 2) a variety of phosphotriesters that include dialkylphosphotriesters, diarylphosphotriesters, glycolate and lactate phosphotriesters, phosphotriester approaches using simultaneous enzymatic and chemical hydrolysis of bis(4-acyloxybenzyl) esters, bis (S-acyl-2-thioethyl) (SATE) esters, cyclosaligenyl prodrugs, glycosyl phosphotriesters, and steroidal phosphotriesters, 3) phosphoramidate derivatives, 4) dinucleoside phosphate derivatives that possess a second anti-HIV moiety such as AZT-P-ddA, AZT-Pddl, AZTP2AZT, AZTP2ACV), and 5) 5'-hydrogen phosphonate and 5'methylene phosphonate derivatives of AZT. In these prodrugs, the conjugating moiety is linked to AZT via a 5'O-ester or 5'-O-phosphate group. 5'-O-Substituted AZT prodrugs have been designed with the objectives of improving anti-HIV activity, enhancing blood-brain barrier penetration, modifying pharmacokinetic properties to increase plasma half-life and improving drug delivery with respect to site-specific targeting or drug localization. Bypassing the first phosphorylation step, regulating transport and conferring sustained release of AZT prolong its duration of action, decrease toxicity and improve patient acceptability. The properties of these prodrugs and their anti-HIV activities are now reviewed.

Cyclooxygenase is the key enzyme in the biosynthesis of prostanoids, biologically active substances that are involved in several physio- logical processes but also in pathological conditions such as inflammation. Since ten years now, it is well known that this enzyme exists under two forms: a constitutive (COX-1) and an inducible form (COX-2). Both enzymes are sensitive to inhibition by conventional nonsteroidal anti-inflammatory drugs (NSAIDs). Observations that COX-1, involved in several homeostatic processes, played a housekeeping role while COX-2 expression was associated with inflammation and other pathologies such as cancer proliferation have led to the development of COX-2 selective inhibitors in order to reduce the classical side-effects, of which gastric irritation is the most common, associated with the use of conventional NSAIDs.

GABA is the major inhibitory neurotransmitter in the brain and GABA re- uptake from the synaptic cleft is one important mechanism in the regulation of GABA activity. Inhibition of the re-uptake of GABA by potent and selective inhibitors of the GABA transporter enhances GABA activity. This property can be used therapeutically in for instance epilepsy or psychiatric disorders. In this paper putative structures of the GABA transporter, its mechanism of action, the progress made in the search for its amino acids involved in the binding of substrates and the SAR of inhibitors acting at the transporter will be discussed. To date only high selective GAT-1 inhibitors are available. These compounds are lipophilic, derivatives of (R)-nipecotic acid and guvacine. The most potent inhibitors of the cloned human GAT-1 are NNC-711 (IC50 = 0.04 uM) and tiagabine (IC50 = 0.07 uM). A diheteroarylvinyloxy analogue of tiagabine, 5 times more potent than tiagabine, has been reported recently. For the GAT-2, GAT-3 and BGT-1 subtypes only compounds with a small preference for one of the subtypes have been published.

A perspective is offered on the recent development of Src-homology 2 (SH2) antagonists of Src, Grb2 and ZAP-70. Inhibiting Src SH2 is believed to be a potentially attractive way of regulating bone resorption. Grb2 SH2 has been shown to be an important component of the mitogenic ras pathway; and thus might be of utility in cancer research. ZAP-70 is a tyrosine kinase that is expressed solely in T-cells and natural killer cells. Since inhibition of the tandem SH2 domains of ZAP-70 has been shown to block T-cell proliferation, antagonists for this particular protein could have implications in immune suppression. The emphasis of the article is placed on the structure-based design, synthesis and biological activity of a number of newly reported SH2 antagonists in each of the three areas.