Polyfluoroaromatic stavudine (d4T) ProTides exhibit enhanced anti-HIV activity.

Human Immunodeficiency Virus (HIV) damages the immune system and leads to the life-threatening acquired immunodeficiency syndrome (AIDS). Despite the advances in the field of antiretroviral treatment, HIV remains a major public health challenge. Nucleosides represent a prominent chemotherapeutic class for treating viruses, however their cellular uptake, kinase-mediated activation and catabolism are limiting factors. Herein, we report the synthesis and in vitro evaluation of stavudine (d4T) ProTides containing polyfluorinated aryl groups against two strains; HIV-1 (IIIB) and HIV-2 (ROD). ProTide 5d containing a meta-substituted pentafluorosulfanyl (3-SF5) aryl group showed superior antiviral activity over the parent d4T and the nonfluorinated analogue 5a. ProTide 5d has low nanomolar antiviral activity; (IC50 = 30 nM, HIV-1) and (IC50 = 36 nM, HIV-2) which is over tenfold more potent than d4T. Interestingly, ProTide 5d showed a significantly high selectivity indices with SI = 1753 (HIV-1) and 1461 (HIV-2) which is more than twice that of the d4T. All ProTides were screened in wild type as well as thymidine kinase deficient (TK-) cells. Enzymatic activation of ProTide 5d using carboxypeptidase Y enzyme and monitored using both 31P and 19F NMR is presented.

Reaction of Thymidine with Hypobromous Acid in Phosphate Buffer.

When thymidine was treated with hypobromous acid (HOBr) in 100 mM phosphate buffer at pH 7.2, two major product peaks appeared in the HPLC chromatogram. The products in each peak were identified by NMR and MS as two isomers of 5-hydroxy-5,6-dihydrothymidine-6-phosphate (a novel compound) and two isomers of 5,6-dihydroxy-5,6-dihydrothymidine (thymidine glycol) with comparable yields. 5-Hydroxy-5,6-dihydrothymidine-6-phosphate was relatively stable, and decomposed with a half-life of 32 h at pH 7.2 and 37°C generating thymidine glycol. The results suggest that 5-hydroxy-5,6-dihydrothymidine-6-phosphate in addition to thymidine glycol may have importance for mutagenesis by the reaction of HOBr with thymine residues in nucleotides and DNA.

Synthesis, Hydrolysis, and Protonation-Promoted Intramolecular Reductive Breakdown of Potential NRTIs: Stavudine α-P-Borano-γ-P-N-L-tryptophanyltriphosphates.

Phosphorus-modified prodrugs of dideoxynucleoside triphosphates (ddNTPs) have shown promise as pronucleotide strategies for improving antiviral activity compared to their parent dideoxynucleosides. Borane modified NTPs offer a promising choice as nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs). However, the availability of α-P-borano-γ-P-substituted NTP analogs remains limited due to challenges with synthesis and purification. Here, we report the chemical synthesis and stability of a new potential class of NRTI prodrugs: stavudine (d4T) 5'-α-P-borano-γ-P-N-L-tryptophanyltriphosphates. One-pot synthesis of these compounds was achieved via a modified cyclic trimetaphosphate approach. Pure Rp and Sp diastereomers were obtained after HPLC separation. Based on LC-MS analysis, we report degradation pathways, half-lives (5-36 days) and mechanisms arising from structural differences to generate the corresponding borano tri- and di-phosphates, and H-phosphonate, via several parallel routes in buffer at physiologically relevant pH and temperature. Here, the major hydrolysis products, d4T α-P-boranotriphosphate Rp and Sp isomers, were isolated by HPLC and identified with spectral data. We first propose that one of the major degradation products, d4T H-phosphonate, was generated from the d4T pronucleotides via a protonation-promoted intramolecular reduction followed by a second step nucleophilic attack. This report could provide valuable information for pronucleotide-based drug design in terms of selective release of target nucleotides.

Synthesis, and in vitro enzymatic and antiviral evaluation of d4T polyphosphate derivatives as chain terminators.

A series of d4T di- or triphosphate derivatives have been synthesized and evaluated as effective substrates for HIV-1 RT, and also tested for their in vitro anti-HIV activity. The steady-state kinetic study of compounds 1-4 in an enzymatic incorporation assay by HIV-1 RT follows Michaelis-Menten profile. In addition, compounds 2-4 are able to inhibit HIV-1 replication to the same extent as d4T and d4TMP in MT-4 cells, as well as in CEM/0 cells and CEM/TK(-) cells. The data suggests that these d4T polyphosphate derivatives are hydrolyzed to d4T and rephosphorylated to d4TTP before exerting their antiviral activity.

Diastereoselective synthesis of cyclosaligenyl-nucleosyl-phosphotriesters.

A diastereoselective synthesis of cycloSal-phosphotriesters (cycloSal=cycloSaligenyl) based on chiral auxiliaries has been developed that allows the synthesis of single diastereomers of the cycloSal-pronucleotides. In previously described synthesis routes, the cycloSal-compounds were always obtained as 1:1 diastereomeric mixtures that could be separated in only rare cases. However, it was shown that the diastereomers have different antiviral activity, toxicity, and hydrolysis stabilities. Here, first a chiral thiazoline derivative was used to prepare nonsubstituted and 5-methyl-cycloSal-phosphotriesters in 48 and ≥95% de (de=diastereomeric excess). However, this approach failed to give the important group of 3-substituted cycloSal-nucleotides. Therefore, two other chiral groups were discovered that allowed the synthesis of (R(P))- and (S(P))-3-methyl-cycloSal-phosphotriesters as well. The antiviral activity was found to be five- to 20-fold different between the two individual diastereomers, which proved the importance of this approach.

Synthesis and biological evaluation of fatty acyl ester derivatives of 2',3'-didehydro-2',3'-dideoxythymidine.

A number of 5'-O-fatty acyl derivatives of 2',3'-didehydro-2',3'-dideoxythymidine (stavudine, d4T) were synthesized and evaluated for anti-HIV activities against cell-free and cell-associated virus, cellular cytotoxicity, and cellular uptake studies. The conjugates were found to be more potent than d4T. Among these conjugates, 5'-O-12-azidododecanoyl derivative of d4T (2), displaying EC(50) = 3.1-22.4 µM, showed 4- to 9-fold higher activities than d4T against cell-free and cell-associated virus. Cellular uptake studies were conducted on CCRF-CEM cell line using 5(6)-carboxyfluorescein derivatives of d4T attached through ß-alanine (9) or 12-aminododecanoic acid (10) as linkers. The fluorescein-substituted analog of d4T with long chain length (10) showed 12- to 15-fold higher cellular uptake profile than the corresponding analog with short chain length (9). These studies reveal that conjugation of fatty acids to d4T enhances the cellular uptake and anti-HIV activity of stavudine.

Synthesis, nanosizing and in vitro drug release of a novel anti-HIV polymeric prodrug: chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate.

A novel approach to improve the antiviral efficacy of nucleoside reverse transcriptase inhibitors (NRTIs) and reduce their side effects was developed by constructing a nanosized NRTI monophosphate-polymer conjugate using d4T as a model NRTI. Firstly, a novel chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate with a phosphoramidate linkage was efficiently synthesized through Atherton-Todd reaction under mild conditions. The anti-HIV activity and cytotoxicity of the polymeric conjugate were evaluated in MT4 cell line. Then the conjugate nanoparticles were prepared by the process of ionotropic gelation between TPP and chitosan-d4T conjugate to improve their delivery to viral reservoirs, and their physicochemical properties were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS) techniques and X-ray diffraction (XRD). In vitro drug release studies in pH 1.1 and pH 7.4 suggested that both chitosan-d4T conjugate and its nanoparticles prefer to release d4T 5'-(O-isopropyl) monophosphate than free d4T for prolonged periods, which resulted in the enhancement of anti-HIV selectivity of the polymeric conjugate relative to free d4T due to bypassing the metabolic bottleneck of monophosphorylation. Additionally, the crosslinked conjugate nanoparticles can prevent the coupled drug from leaking out of the nanoparticles before entering the target viral reservoirs and provide a mild sustained release of d4T 5'-(O-isopropyl) monophosphate without the burst release. The results suggested that this kind of chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate nano-prodrugs may be used as a targeting and sustained polymeric prodrugs for improving therapy efficacy and reducing side effects in antiretroviral treatment.

The synthesis and NMR investigation on novel boron derivatives of stavudine.

Preparation and spectroscopic properties of novel boron-containing derivatives of anti-HIV agent stavudine are presented, The new compounds, (5'-O-(4,4,5,5-tetramethyl-1,3,2-dioxaboronate)-2'-3'-didehydro-2'-3'-dideoxythymidine and 5'-O-(dihydroxyboronate)-2'-3'-didehydro-2'-3'-dideoxythymidine), were prepared by direct reaction between stavudine and reagents containing BH moieties - pinacolborane and borane-dimethylsulfide complexes, respectively. The boron coordination equilibrium of those compounds was analyzed by water titration monitored by NMR. Results of the DFT calculations and NMR experiments pointed to structural and electronic similarity of tetrahedral boron complexes to phosphate group.

Novel synthesis and in vitro drug release of polymeric prodrug: Chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate.

A novel approach to synthesize chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate was developed. Chitosan-d4T monophosphate prodrug with a phosphoramidate linkage was efficiently synthesized through Atherton-Todd reaction. In vitro drug release studies in pH 1.1 and 7.4 indicated that chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate prefers to release the d4T 5'-(O-isopropyl)monophosphate than free d4T for a prolonged period. The results suggested that chitosan-O-isopropyl-5'-O-d4T monophosphate conjugate may be used as a sustained polymeric prodrug for improving therapy efficacy and reducing side effects in antiretroviral treatment.

Enzymatically activated cycloSal-d4T-monophosphates: The third generation of cycloSal-pronucleotides.

The third generation of cycloSal-pronucleotides, 5-diacetoxymethyl-cycloSal-d4T-monophosphates (5-di-AM-cycloSal-d4TMPs), is reported as a new class of "lock-in"-modified cycloSal-pronucleotides. These compounds bear an esterase-cleavable geminal dicarboxylate (acylal) attached to the aromatic ring of the saligenyl unit. The conversion into a strong acceptor group (aldehyde) leads to a strong decrease in hydrolytic stability. As a consequence, a fast release of a nucleoside monophosphate (i.e., d4TMP) follows. The concept of this enzymatic activation is proven by hydrolysis studies in phosphate buffer, cell extracts, and human serum. These investigations showed the conversion of the acylal group into a polar aldehyde by enzymatic cleavage. Besides, antiviral activities against HIV are presented.

Bis-cycloSal-d4T-monophosphates: drugs that deliver two molecules of bioactive nucleotides.

Bis-cycloSal-d4T-monophosphates have been synthesized as potentially anti-HIV active "dimeric" prodrugs of 2',3'-dideoxy-2',3'-didehydrothymidine monophosphate (d4TMP). These pronucleotides display a mask-drug ratio of 1:2, a novelty in the field of pronucleotides. Both bis-cycloSal-d4TMP 6 and bis-5-methyl-cycloSal-d4TMP 7 showed increased hydrolytic stability as compared to their "monomeric" counterparts and a completely selective hydrolytic release of d4TMP. The hydrolysis pathway was investigated via 31P NMR spectroscopy. Moreover, due to the steric bulkiness, compound 6 already displayed strongly reduced inhibitor potency toward human butyrylcholinesterase (BChE), while compound 7 turned out to be devoid of any inhibitory activity against BChE. Partial separation of the diastereomeric mixture of 6 revealed strong dependence of the pronucleotides' properties on the stereochemistry at the phosphorus centers. Both 6 and 7 showed good activity against HIV-1 and HIV-2 in wild-type CEM cells in vitro. These compounds were significantly more potent than the parent nucleoside d4T 1 in HIV-2-infected TK-deficient CEM cells, indicating an efficient TK-bypass.

An alternative synthetic method for 4'-C-ethynylstavudine by means of nucleophilic substitution of 4'-benzoyloxythymine nucleoside.

For the synthesis of 2',3' -didehydro-3' -deoxy-4' -C-ethynylthymidine (8: 4' -Ed4T), a recently reported promising anti-HIV agent, a new approach was developed. Since treatment of 1-(2,5-dideoxy-beta-l-glycero-pent-4-enofuranosyl)thymine with Pb(OBz)4 allowed the introduction of a 4'-benzoyloxy leaving group, nucleophilic substitution at the 4' -position became feasible for the first time. Thus, reaction between the 4'-benzoyloxy derivative (11) and Me3SiC identical with CAl(Et)Cl as a nucleophile led to the isolation of the desired 4'-"down"-ethynyl derivative (15) stereoselectively in 62% yield.

Synthesis and anti-human immunodeficiency virus activity of 4'-branched (+/-)-4'-thiostavudines.

Motivated by our recent finding that 4'-ethynylstavudine (4) is a promising anti-human immunodeficiency virus type 1 (HIV-1) agent, we synthesized its 4'-thio analogue, as well as other 4'-thiostavudines having a carbon substituent at the 4'-position, as racemates in this study. Methyl 3-oxo-tetrahydrothiophen-2-carboxylate (5) was used as a starting material to construct the requisite 4-thiofuranoid glycal (13). Introduction of a thymine base was carried out by an electrophilic addition reaction to 13 using N-iodosuccinimide (NIS) and bis(trimethylsilyl)thymine. The desired beta-anomer (16beta) obtained as a major product in this reaction underwent ready elimination with activated Zn to give the 4'-carbomethoxy derivative (18). By using 18 as a common intermediate, 4'-carbon-substituted (CH2OH, CO2Me, CONH2, CH=CH2, CN, and C(triple bond)CH) 4'-thiostavudines were prepared. Among these six compounds, 4'-cyano (28) and 4'-ethynyl (29) analogues were found to show inhibitory activity against HIV-1 with ED50 values of 7.6 and 0.74 microM, respectively. The activity of 29 was comparable to that of stavudine, but 29 was not as active as 4. Optical resolution of 29 was briefly examined.

Application of the cycloSal-prodrug approach for improving the biological potential of phosphorylated biomolecules.

Pronucleotides represent a promising tool to improve the biological activity of nucleoside analogs in antiviral and cancer chemotherapy. The cycloSal-approach is one of several conceptually different pronucleotide systems. This approach can be applied to various nucleoside analogs. A salicyl alcohol as a cyclic bifunctional masking unit is used, and shown to afford a chemically driven release of the particular nucleotide from the lipophilic phosphate triester precursor molecule. A conceptual extension of the cycloSal-approach results in the design of "lock-in"-cycloSal-derivatives. The cycloSal-approach is not restricted to the delivery of bioactive nucleotides but also useful for the intracellular delivery of hexose-1-phosphates.

Protease-mediated enzymatic hydrolysis and activation of aryl phosphoramidate derivatives of stavudine.

Several proteases are capable of hydrolyzing the aryl substituted phosphoramidate derivatives of stavudine resulting in the formation of the active metabolite, alaninyl d4T monophosphate. Subtilisin Protease A, Subtilisin Griseus, Subtilisin Carlsberg, Papaya, Bacillus were amongst the most effective proteases in hydrolyzing stavudine derivatives and specificity of their activity was confirmed using several protease inhibitors to block the hydrolysis of these phosphoramidate derivatives. We found that these proteases exhibit chiral selectivity at the phosphorus center of stavudine derivatives. Our results indicate that cellular proteases may be responsible for the activation of these phosphoramidate derivatives. In addition, we show that the enzymatic hydrolysis takes place at the carboxymethyl ester side chain of these pro-drugs and the direct attack on the phosphorus center by these enzymes does not occur. Finally, we describe a novel activation pathway hitherto unknown for the activation and viral inhibitory characteristic shown by these phosphoramidate derivatives of stavudine.

Synthesis of (+/-)-4'-ethynyl and 4'-cyano carbocyclic analogues of stavudine (d4T).

The synthesis of (+/-)-4'-ethynyl (8) and 4'-cyano (9) carbocyclic analogues of the anti-HIV agent stavudine (5, d4T) is reported. The carbocyclic unit (16) was constructed from readily available beta-keto ester 10. The ethynyl or cyano group of 8 and 9 were prepared, after the introduction of thymine base to 16, by manipulation of the ester function. Evaluation of the anti-HIV activity of 8 and 9 was also carried out.

A new approach to the synthesis of 4'-carbon-substituted nucleosides: development of a highly active anti-HIV agent 2', 3'-didehydro-3'-deoxy-4'-ethynylthymidine.

Oxidation of 3'-O-TBDMS-4',5-unsaturated thymidine 3 with dimethyldioxirane (DMDO) allowed the isolation of the epoxide 4. Upon reacting with organosilicon reagents in the presence of SnCl4, 4 underwent stereoselective ring opening to give 4'-alpha-allyl (6), 4'-alpha-(2-bromoallyl) (7), 4'-alpha-(cyclopenten-3-yl) (8), and 4'-alpha-cyano (9) derivatives of thymidine. Reactions of the 3'-epimer 12 with organoaluminum reagents gave 4'-alpha-methyl (13), 4'-alpha-vinyl (14), and 4'-alpha-ethynyl (15) analogues. Compounds 13-15 were transformed into corresponding 2',3'-didehydro-3'-deoxy derivatives. Evaluation of their ability to inhibit the replication of HIV in cell culture showed that 4'-ethynyl-d4T (19) is more potent and less toxic than the parent compound d4T.

"Lock-in" modified cyclosal nucleotides--the second generation of cyclosal prodrugs.

A new generation of cycloSal-pronucleotides is presented. CycloSal-d4TMPs have been modified by introduction of an esterase-cleavable site in order to trap them inside cells. Hydrolysis studies in different media (PBS, CEM/0- and liver extracts) and anti-HIV evaluation of separated diastereomers revealed unexpected differences between the isomers.

Synthesis and anti-HIV activity of beta-D-3'-azido-2',3'-unsaturated nucleosides and beta-D-3'-azido-3'-deoxyribofuranosylnucleosides.

Since the discovery of 3'-azido-3'-deoxythymidine (AZT) and 2',3'-didehydro-2',3'-dideoxythymidine (d4T) as potent and selective inhibitors of the replication of human immunodeficiency virus (HIV), there has been a growing interest for the synthesis of 2',3'-didehydro-2',3'dideoxynucleosides with electron withdrawing groups on the sugar moiety. Here we described an efficient method for the synthesis of such nucleoside analogs bearing structural features of both AZT and d4T The key intermediate, 3-azido-1,2-bis-O-acetyl-5-O-benzoyl-3-deoxy-D-ribofuranose, 5 was synthesized from commercially available D-xylose in five steps, from which a series of pyrimidine and purine nucleosides were synthesized in high yields. The resultant protected nucleosides were converted to target nucleosides using appropriate chemical modifications. The final nucleosides were evaluated as potential anti-HIV agents.

A Claisen approach to 4'-Ed4T.

An efficient, stereoselective synthesis of 4'-Ed4T is demonstrated. The synthesis is highlighted by a regioselective TMSOTf-mediated acetal opening, a Claisen rearrangement to set the key 4'-stereocenter as well as the olefin, and a one-pot nonaflation/elimination to deliver the alkyne moiety. The synthesis proceeds in eight steps from 5-methyluridine and occurs in 37% overall yield.