## 3 Anti-HIV Pronucleotides-Nucleotide Reverse-Transcriptase Inhibitors (NttTIs)
### Triphosphate Esters of 2',3'-Dideoxynucleosides
Pronucleotides of type **39c** (R\({}^{1}\) = Et or Bu; R\({}^{2}\) = C\({}_{16}\)H\({}_{33}\)) but bearing other than d4T antiviral nucleoside residues were also prepared, and showed similar or even slightly better activities against HIV-1 and HIV-2 than the parent nucleosides in both TK\({}^{+}\) and TK' cells [118]. Despite the promising results obtained with pronucleotides Tri_PPPro_ of type **39** (bearing the benzyl groups derivatized with both carboxy and/or carbonate ester groups),detailed analyses revealed some drawbacks of these constructs. The stability studies in enzymatic media showed that although the compounds **39** were admittedly liberated of the desired NTPs, their rapid intracellular dephosphorylation compromised their pronucleotide mode of action. Moreover, a careful scrutiny of the possible sources of the observed cytotoxicity strongly suggested that the released ddNTTPs were not only substrates for the viral reverse transcriptase (RT), but also to some extent for cellular polymerases, particularly for the mitochondrial DNA polymerase \(\gamma\)[120]. To remedy these problems, [PERSON] et al. developed another type of unsymmetrical triphosphate pronucleotides **43** (Figure 9) that contained only one biodegradable acyloxbenzyl group, as in the previous constructs (R = C1-C15), in combination with the simple alkyls phosphate esters of different chain length (C4, C15, C18). The aim of this was, on the one hand, to ensure the adequate lipophilicity of the pronucleotides, which would enable the efficient crossing of cell membranes, and, on the other hand, to protect the triphosphate residue against dephosphorylation by the presence of a more difficult-to-cleave \(\gamma\)-alkyl group. It was also expected that the presence of the \(\gamma\)-alkyl group would prevent the binding of such triphosphate analogs by cellular polymerases, but not by the more promiscuous viral RT enzyme. For the synthesis of prodrugs **43**, the same H-phosphonate method (Scheme 22) as for prodrugs **39b-d** (yields 33-63%) was used, and the expected products of their cellular metabolism, \(\gamma\)-alkyl-NTPs **44**, were obtained via \(\beta\)-elimination of the cyanoethyl group. The synthesized pronucleotides **43** were found to successfully enter the cells, followed by a selective cleavage of the benzyl group. The formed \(\gamma\)-alkyl-NTPs **44** appeared to be stable in cell extracts, as expected, and acted as substrates for the viral RT only. This selectivity can form the basis for fighting viral infection in the body without interfering with the vital functions of the cell. Almost all pronucleotides **43** obtained were slightly more active against HIV-1 and HIV-2 than the parent nucleoside d4T, and some of them retained their full activity in the TK' cells. The \(\gamma\)-alkyl-NTPs **44** were similarly active in normal CEM/0 cells, while their anti-HIV activity in TK' cells varied dramatically depending on the alkyl chain length. Thus, for derivatives **44** with R = C4 or C11, no anti-HIV activity was observed, while for R = C18, the antiviral activity became very high (EC\({}_{50}\) = 0.05 uM) [120].