The biochemical pharmacology of (2′-R)-chloropentostatin, a novel inhibitor of adenosine deaminase

1. 1.|2′-Chloropentostatin is a new inhibitor of adenosine deaminase isolated from the fermentation broth of an unidentified actinomycete, ATCC 39365. It contains the aglycone of coformycin, i.e. 3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepin-8-ol, coupled to the unusual carbohydrate, 2′-chloro-2′-deoxyribose. 2. 2.|2′-Chloropentostatin is a slightly weaker inhibitor of rat and human adenosine deaminases than coformycin, and considerably weaker than pentostatin. 3. 3.|Unlike pentostatin, which appears to undergo a two-stage interaction with adenosine deaminase, 2′-chloropentostatin forms a single enzyme-inhibitor complex. 4. 4.|The enzyme-inhibitor complex between adenosine deaminase and 2′-chloropentostatin was much more rapidly dissociable than the complex with pentostatin. The complex between adenosine deaminase and 2′-chloropentostatin dissociated with a half-life of approximately 3 hr, compared with 68 hr for the complex between adenosine deaminase and pentostatin. 5. 5.|2′-Chloropentostatin, at concentrations up to 10 micromolar, did not cause significant inhibition of growth of WI-L2 human B-cell lymphoblasts, or of CCRF-CEM human T-cell lymphoblasts in culture. However, it greatly potentiated the inhibitory potency of adenosine, 2′-deoxyadenosine, or arabinosyladenine towards these cell lines. This potentiating effect was equipotent for 2′-chloropentostatin and pentostatin. 6. 6.|T-cells (CCRF-CEM) were much more sensitive to the inhibitory effect of combinations of adenosine or 2′-deoxyadenosine with 2′-chloropentostatin or pentostatin than were B-cells (WI-L2). 7. 7.|Pentostatin and 2′-chloropentostatin had no significant antitumor activity against mouse leukemia L1210 in vivo. However, these adenosine deaminase inhibitors, at nontoxic doses, greatly potentiated the antitumor activity of ara-A 5′-phosphate. 2′-Chloropentostatin was somewhat more active in this regard than was pentostatin.