Hydrolysis and solvent-dependent 2'----5'and 3'----5' acyl migration in prodrugs of 9-beta-D-arabinofuranosyladenine

2-Ester Prodrugs of the Varicella-Zoster Antiviral Agent, 6-Methoxypurine Arabinoside

6-Methoxypurine arabinoside [9-(β-D-arabinofuranosyl)-6-methoxy-9H-purine; 1] is a potent and selective agent against varicella-zoster virus in vitro. Studies in the rat indicated that extensive first pass metabolism of this nucleoside occurred after oral dosing, presumably from high levels of adenosine deaminase in the intestine. Only 3.8% of an oral dose of 6-methoxypurine arabinoside was recovered in the urine. In an attempt to improve these unfavourable pharmacokinetics, a series of twenty-five 2′-esters of 6-methoxypurine arabinoside were prepared either by direct acylation or by a protection, acylation, and deprotection sequence. These esters were evaluated in the rat for their suitability as prodrugs on the basis of urinary recovery of 1 after oral dosing. The straight-chain aliphatic esters, but not the branched chain or aromatic esters, improved this urinary recovery. The 2′-valerate, hexanoate, heptanoate, and octanoate esters (2d–g, respectively) were the most effective, with urinary recoveries of 1 above 15%. The water solubility of the valerate ester was high (107 mM at 37°C and pH 7.2), but solubilities progressively decreased with the longer chain esters. The combination of high water solubility and improved metabolic stability upon oral dosing made 2d the prodrug of choice for both oral and intravenous dosing.

Mechanism of chemical degradation and determination of solubility by kinetic modeling of the highly unstable sesquiterpene lactone nobilin in different media

The objective of this work was first to investigate the chemical degradation of the sesquiterpene lactone nobilin and determine its solubility under conditions of concurrent degradation for partially amorphous starting material; second, to determine the effect of biorelevant media used in the in vitro measurement of intestinal absorption on degradation and solubility of nobilin. Purely aqueous medium (aq-TMCaco ), fasted and fed state simulated intestinal fluid (FaSSIF-TMCaco and FeSSIF-TMCaco ), and two liposomal formulations (LiposomesFaSSIF and LiposomesFeSSIF ) with the same lipid concentration as FaSSIF-TMCaco and FeSSIF-TMCaco were used. Degradation products were identified by nuclear magnetic resonance and X-ray crystallography and the order of reaction kinetics was determined. Solubility was deduced with a mathematical model encompassing dissolution, degradation, and reprecipitation kinetics that took into account particle size distribution of the solid material. Degradation mechanism of nobilin involved water-catalyzed opening of the lactone ring and transannular cyclization resulting in five degradation products. Degradation followed first-order kinetics in aq-TMCaco and FaSSIF-TMCaco , and higher-order kinetics in FeSSIF-TMCaco and the two liposomal formulations, whereas degradation in the latter media was diminished. Solubility of nobilin increased in the order: aq-TMCaco < FaSSIF-TMCaco , < LiposomesFaSSIF < FeSSIF-TMCaco < LiposomesFeSSIF . Improvement of stability and solubility was consistent with the incorporation of the nobilin molecule into colloidal lipid particles. The developed kinetic model is proposed to be a useful tool for deducing solubility under dynamic conditions.

Synthetic approaches to a mononucleotide prodrug of cytarabine

Synthetic pathways to a mononucleotide prodrug of cytarabine (Ara-C) bearing S-pivaloyl-2-thioethyl (tBuSATE) groups, as biolabile phosphate protections, are reported. Using a common phosphoramidite approach, two different kinds of nucleoside protecting groups have been investigated. During this study, we observed an intermolecular migration of the Boc protecting group in the course of the tert-butyldimethylsilyl ether cleavage using tetrabutyl ammonium fluoride.