Effects of (R)‐deoxycoformycin (pentostatin) on intrauterine nucleoside catabolism and embryo viability in the pregnant mouse

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Abstract
The viability of early mouse embryos is acutely sensitive to (R)‐deoxycoformycin (pentostatin), a tight‐binding inhibitor of adenosine deaminase (ADA). Previous studies have shown that a single 5‐mg/kg dose on day 7 (plug = day 0) of gestation fully inhibits uteroplacental ADA activity within 0.5 h; causes massive cell death in the neural plate and primary mesenchyme by 6 h, major craniofacial anomalies by day 10, and resorption by day 12 [Knudsen] et al., '89; Airhart et al., '91). The present study has examined further the developmental toxicity and early effects of this inhibitor on ADA metabolism. (R)‐Deoxycoformycin was administered to pregnant CD‐1 (ICR) mice as a single intraperitoneal dose of 0.5‐10 mg/kg total body weight on days 6–11 of gestation. The major adverse effect, early resorption, was dose dependent and specific to day 7–8 exposure. Treatment with 5 mg/kg on day 7 resulted in 85% resorptions, 15% malformations, and a 24% reduction in mean fetal weight, whereas the same dose of (S)‐deoxycoformycin had no effect. Levels of adenosine and 2 ′‐deoxyadenosine, which are the endogenous substrates of ADA, were monitored in the embryo/decidual unit (E/D) by reversed‐phase high‐performance liquid chromatography (RP‐HPLC). In response to the inhibitor, both nucleosides increased transiently in the antimesometrial compartment (antimesometrial decidua + embryo). Peak levels (Cmax) of adenosine and 2 ′‐deoxyadenosine were dose dependent over the range tested (0.05‐10 mg/kg). Exposure to 5 mg/kg on day 7 raised adenosine levels within 0.5 h to 42‐fold over the basal level of 0.06 nmol/mg protein. There was an even stronger effect on 2 ′‐doexyadenosine levels, which were elevated 674‐fold over the detection limit of 0.0005 nmol/mg protein. Direct exposure to the inhibitor in serum‐free E/D culture produced similar results: 50 μM (R)‐deoxycoformycin within 1 h raised adenosine levels 26‐fold and 2 ′‐deoxyadenosine levels 410‐fold. In vivo studies also showed a general correlation between embryolethality and the length of adenine nucleoside pool expansion, apparent for exposure on day 7, 8, or 9 but not on day 6, suggesting that the embryo becomes sensitive to adenosine or 2 ′‐deoxyadenosine once the neural plate has formed.
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