Studies on (8-14C)adenosine-5'-carboxylic acid ethyl ester. I. Metabolic fate in the dog, rat and mouse

Metabolism of 8-(methylthio)cyclic 3',5'-adenosine monophosphate by rats and dogs after oral or intravenous dosing and in vitro by subcellular preparations of dog liver

8-(Methylthio-14C or -35S)cyclic 3',5'-adenosine monophosphate (I) was given intravenously to rats (5 mg/kg) and orally and intravenously to dogs (0.25, 2.5, or 50 mg/kg). Oral doses were absorbed well but slowly. Plasma half-lives in dogs were about 3 hr after oral or intravenous doses of 0.25 or 2.5 mg/kg and ranged from 5 to 12 hr after oral or intravenous doseas of 50 mg/kg. Plasma glucose and insulin concentrations in dogs were increased by oral or intravenous doses of the compound. Regardless of the route, excretion of radioactivity by rats and dogs at all doses was chiefly in the urine (74-87% of the dose); the remainder was excreted in the feces or bile. Compound I was rapidly distributed to most tissues of dogs but entered the brain and certain portions of the eye slowly and to a limited extent. Urine and plasma of dogs and urine of rats contained I, 8-(methylthio)adenosine, and at least two other unidentified metabolites. Compound I and cyclic 3',5'-adenosine monophosphate were metabolized in vitro by the soluble fraction of dog liver to form 8-(methylthio)adenosine-5'-monophosphate and adenosine-5'-monophosphate, respectively. These compounds were further converted to 8-(methylthio)adenosine and adenosine, respectively. Compound I was metabolized in vitro more slowly than cyclic 3',5'-adenosine monophosphate.

Cardiovascular effects of nucleoside analogs

Short-chain aliphatic esters and amides of adenosine-5'-carboxylic acid caused marked increases in coronary sinus oxygen tension (PO2) in the dog; the amides were generally more potent, causing additionally marked hypotension and tachycardia. The hypotensive effect was observed also in the spontaneously hypertensive rat. That the increase in coronary sinus PO2 paralleled an increase in coronary flow was verified with ethyl adenosine-5'-carboxylate hydrochloride. This compound also increased the reactive hyperemic response. Aminophylline blocked the increase in coronary flow. A representative amide and ester were very poor substrates for adenosine and adenylate deaminase in vitro; the amide exhibited a weak inhibitor effect on the enzymic activities while the ester was inactive. The observations that the compounds (1) cause marked pharmacological effects within seconds after intravenous administration, (2) are blocked by aminophylline like adenosine, (3) are not deaminated significantly in vitro by either adenosine or adenylate deaminase, and (4) cannot be phosphorylated at the 5' terminus because the 5'-OH has been removed chemically, support the hypothesis that they are acting directly on an "adenosine receptor" and have a prolonged duration of action because they are not metabolized significantly by the normal physiological pathways of adenosine degradation.