Enzymatic and cellular study of a serotonin N-acetyltransferase phosphopantetheine-based prodrug

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) regulates the daily rhythm in the production of melatonin and is therefore an attractive target for pharmacologic modulation of the synthesis of this hormone. Previously prepared bisubstrate analogs show potent inhibition of AANAT but have unfavorable pharmacokinetic properties due to the presence of phosphate groups which prevents transfer across the plasma membrane. Here, we examine a bis-pivaloyloxymethylene (POM)-tryptamine-phosphopantetheine prodrug (2) and its biotransformations in vitro by homogenates and pineal cells. Compound 2 is an efficient porcine liver esterase substrate for POM cleavage in vitro although cyclization of the phosphate moiety is a potential side product. Tryptamine phosphopantetheine (3) is converted to tryptamine-coenzyme A (CoA) bisubstrate analog (1) by human phosphoribosyl pyrophosphate amidotransferase (PPAT) and dephosphocoenzyme A kinase (DPCK) in vitro. Compound 2 was found to inhibit melatonin production in rat pineal cell culture. It was also found that the POM groups are readily removed to generate 3; however, further processing to tryptamine-CoA (1) is much slower in pineal extracts or cell culture. Implications for CoA prodrug development based on the strategy used here are discussed.