Biotransformation of 9-beta-D-arabinofurano-syladenine by rat and human erythrocytes.

Studies were performed to test whether 9-/l-D-arabinofuranosyladenine (ara-A) would accumulate in erythrocytes as a result of phosphorylation to the nucleotide level. When C3H]ara-A was incubated with whole blood from rat, monkey or man for 2 hr at 37”, the drug rapidly equilibrated between erythrocytes and plasma but did not concentrate in the cells. Incubation of C3H]ara-A with rat and human erythrocyte lysates for 2 hr followed by chromato~aphic analysis showed that 2-5 per cent of ara-A was converted to nucleotides. In contrast, 10-35 per cent of [‘YY]adenosine was converted to adenine nucteotides under the same conditions. Incubation of C3H]ara-A with human erythrocyte lysates for 18 hr resulted in a conversion of approx. 40 per cent of the labeled drug to nucleotides. Additional chromatography revealed, however, that the nucleotide fraction contained almost no arabinosyl nucleotides. Rather, 90 per cent of the label in the nucleotide fraction was identified as IMP. These results indicate that only a minor amount, if any, of ara-A was phosphorylated by erythrocyte enzymes to yield arabinosyl nucleotides. An alternative pathway converted much of the labeled drug to ribosyl nucleotides via the deamination of ara-A to ara-hypoxanthine, cleavage to hypoxanthine and conversion of the free hypoxanthine to IMP. MANY nucleoside antibiotics require conversion to the corresponding S-phosphates to express antineoplastic or antiviral activity. Conversion to the nucleotide can occur in either host tissues, tumor cells or viruses. ‘*’ Phosphorylation by host tissues will produce a compound active against not only the tumor or virus, but also against proliferating host cells. Ara-A$ is a nucleoside antibiotic having broad spectrum activity against DNA viruses3 but with low toxicity to experimental animals4 and man.5 The low toxicity probably is related to the rapid deamination of ara-A to the less active ara-Hx and its subsequent phosphorolysis to h~oxanth~ne.6,7 Since the Striphosphate of ara-A appears to be the active form,8 the low toxicity also might be due to an inability of normal animal tissues to phosphorylate the drug. Mammalian erythrocytes commonly phosphorylate fraudulent nucleosides resulting in a preferential uptake because the nucleoside is converted to the more ionized * Present address: Department of Pharmacology. Michigan State University, East Lansing, MI 48823, U.S.A. t N. S. F. Undergraduate Research Participant in Medicinal Chemistry, Summer 1970. 1 Abbreviations used: ara-A. 9-P-o-arabinofuranosyladenine; ara-AMP, ara-ADP and ara-ATP, the corresponding 5’-mono-, diand triphosphates of ara-A; ara-Hx, 9-/I-D-arabinofuranosylhypoxanthine; ara-IMP, the corresponding 5’-monophosphate of ara-Hx; Ade, adenine; Hyp, hypoxanthine; PEI, polyethyleneimine.