Chromatographic R Mw and log k' w parameters have been used as alternatives to the shake flask n-octanol/water partition coefficient method to study the lipophilic character of several known pyrimidinic nucleoside derivatives and two novel ones. The R M and log k' values were measured by means of reversed-phase thinlayer chromatography (RP-TLC) and reversed-phase high performance liquid chromatography (RP-HPLC) respectively. Good linear relationships were observed between log P values obtained from the classical shake flask method (log P o/w ) and each one of those obtained by chromatographic techniques (log P RP-TLC and log P RP-HPLC ) and by theoretical calculations using the CLOGP program (log P CLOGP ). The R Mw and log k' w values have proven to be reliable parameters for describing the lipophilic properties of this family of compounds. 855

A solid-phase extraction methodology, followed by high-performance liquid chromatography (HPLC) quantification with UV absorbance detection (lambda = 267 nm), was developed in order to study the stability of 3'-azido-3'-deoxy-5'-O-isonicotinoylthymidine (AZT-Iso), a novel derivative of the antiretroviral AZT, in different matrixes. The half-lives (t1/2) for AZT-Iso were 1.19, 1.13 and 0.30 h for human, rat and rabbit plasma, respectively, and 14.91 and 25.49 h for potassium phosphate buffer (pH 7.4) and human serum albumin solution, respectively. The HPLC method proved to be selective, sensitive and accurate. Good recovery, linearity and precision were achieved using p-fluorophenol as an internal standard. The validity of this method was tested using synthetic mixtures of the intact drug with its decomposition products. In conclusion, the method presented is applicable to in vivo pharmacokinetics studies of AZT-Iso in rats.

Synthesis, solid state characterization and anti HIV-1 activity of 3'-azido-3'-deoxy-5'-O-isonicotinoylthymidine (2), a new prodrug of zidovudine (AZT, 1), are described. Two solid forms of 2 prepared by crystallization from ethyl acetate-petroleum ether (form alpha) and from a melt sample of form alpha (amorphous form) were characterized by X-ray diffractometry, infrared spectroscopy, differential scanning calorimetry (DSC) and thermogravimetry (TGA) techniques. The novel nucleoside exhibited antiviral activity against standard and resistant strain panels of HIV-1 as well as cytotoxicity similar to that of AZT.

Degradation of 3'-azido-3'-deoxy-5'-O-isonicotinoylthymidine (AZT-Iso), an antiretroviral derivative of zidovudine, was investigated in buffer pH 7.4, mu = 300 mOsm at 37, 50 and 60 degrees C, and in water (pH 6.6, 37 degrees C), giving zidovudine (AZT) and isonicotinic acid (INA) as products. The rate constants were determined by reversed-phase HPLC showing pseudo-first-order kinetics related to the residual amount of AZT-Iso. In this way, the studied compound was demonstrated to be 153 times more stable in water than in buffer solution at 37 degrees C. The analytical method was conveniently validated demonstrating to be a rapid and accurate stability-indicating technique. In addition, experimental and theoretical values of pKa were determined.

The pH-solubility behavior and solubility of 3'-azido-3'-deoxy-5'-O-isonicotinoylthymidine (AZT-Iso), an antiretroviral derivative of zidovudine with important biological activity, was studied in water, ethanol, ethanol: water, and n-octanol. The N-pyridine pKa value was determined from its pH-solubility profile, which was in accordance with that of the experimental value of methyl isonicotinate. Also, the ethanol cosolvency in ethanol:water mixtures at 25 degrees C was studied, and log-linear and nonlinear solubilization models were applied to the experimental solubility AZT-Iso data, which allowed us to predict its solubility in those solvent mixtures at a determined content of cosolvent.