Arabinosyl hypoxanthine

Vidarabine is administered only as a topical ophthalmic ointment. It has relatively limited solubility and is not significantly absorbed after application to the eye. Within the tissues, it is rapidly deaminated to its principal metabolite, arabinosyl hypoxanthine, which retains some degree of antiviral activity. 

The main metabolite, arabinosyl hypoxanthine (Ara-Hx) has approximately 1/20 the activity of vidarabine. In renal impairment, the excretion of Ara-Hx is decreased, requiring dose adjustment. Allopurinol, a xanthine oxidase inhibitor, may interfere with the metabolism of vidarabine (see also Figure 18). 

Vidarabine is deaminated quite rapidly by the enzyme adenine deaminase that is usually found in serum and RBC. Interestingly, this enzyme helps in the conversion of this drug , into its principal metabolite termed as arabinosyl hypoxanthine (ara-HX), which displays weak antiviral activity ... 

Vidarabine is deaminated rapidly by adenosine deaminase, which is present in serum and red blood cells. The enzyme converts vidarabine to its principal metabolite, arabinosyl hypoxanthine (ara-HX), which has weak antiviral activity (Fig. 43.8) (58). The half-life of vidarabine is approximately 1 hour, whereas ara-HX has a half-life of 3.5 hours. The drug is detected mostly in the kidney, liver, and spleen, because 50% of it is recovered in the urine as ara-HX. Levels of vidarabine in CSF fluid are 50% of those in the plasma. 

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