Chemical stability of pentostatin (NSC-218321), a cytotoxic and immunosuppressant agent.

Pentostatin, an unusual nucleoside of natural origin, has been used for the treatment of hairy cell leukemia, as an immunosuppressant agent, and as an inhibitor of adenosine deaminase. The studies of the physicochemical properties and solution stability of pentostatin are important to the development of a parenteral formulation for extensive preclinical and clinical testing. Pentostatin displayed apparent pKa values at 25 +/- 0.1 degree C and ionic strength of 0.15 M of 2.03 +/- 0.03 and 5.57 +/- 0.14 (spectrophotometric) and 5.50 +/- 0.02 (potentiometric) for N1 and the amidine nitrogen in the seven-membered ring, respectively, which are the most likely protonation sites. The rates of degradation of pentostatin were determined as a function of pH, buffer concentration, and temperature. In the pH range 1.0-4.0, pentostatin undergoes acid-catalyzed glycosidic cleavage leading to the formation of the base compound, and 2-deoxyribose. A carbonium ion mechanism in which C-N bond cleavage was the rate-determining step was consistent with the data. In the pH range 6.5-10.5, the imine bond at C5 position in pentostatin is hydrolyzed to form the corresponding formamide. Pentostatin hydrolysis in this pH range was independent of pH. At pH greater than 11, pentostatin decomposes to nonchromophoric products probably through multiple-step base-catalyzed hydrolytic mechanisms. Pentostatin appears to be quite stable after reconstitution of a lyophilized experimental dosage form. Care must be taken if pentostatin is extensively diluted with 5% dextrose in water, as pentostatin stability is compromised at pH values less than 5.

5-Quinone derivatives of 2'-deoxyuridine 5'-phosphate: inhibition and inactivation of thymidylate synthase, antitumor cell, and antiviral studies.

Both photochemical aromatic substitution and palladium (0)-catalyzed biaryl coupling reactions have been employed in the synthesis of 5-substituted 2'-deoxyuridines. The former procedure was useful in the preparation of the 3,4-dimethyl-2,5-dimethoxyphenyl derivative 12a and the 3,4,6-trimethyl-2,5-dimethoxyphenyl derivative 12b. The latter reaction was efficient in the preparation of the 2-(3-methyl-1,4-dimethoxynaphthyl) derivative 14. These compounds and their nucleotides (20a-c) were converted to the corresponding quinone nucleosides 19a-c and nucleotides 6-8 by an oxidative demethylation reaction using ceric ammonium nitrate and silver(II) oxide, respectively. The kinetics and products of the reaction of the quinone nucleosides 19a,b with methyl thioglycolate showed rapid addition to the quinone ring in the trisubstituted derivative 19a and somewhat slower redox reactions with the tetrasubstituted quinones 19b and 19c. All six nucleotides had high affinity for the title enzyme from Lactobacillus casei with Ki values ranging from 0.59 to 3.6 microM; the most effective compounds were the dimethyl quinone 6 and the naphthoquinone 8. Somewhat higher inhibitory constants were observed with the quinones against the L1210 enzyme. The dimethyl quinone nucleotide 6 showed time-dependent inactivation (kinact = 0.015 s-1) against the L. casei enzyme, a rate saturation effect, and substrate protection in accord with the kinetic expression for an active-site-directed alkylating agent. The apparent second-order rate of this reaction (2.5 X 10(4) M-1 s-1) is one-twentieth the rate (kcat.) of the normal enzymatic reaction leading to product. None of the compound exhibited sufficient activity in the antitumor cell or antiviral assays to warrant further study.