8-OXO-Cordycepin Is Not a Suitable Substrate for Adenosine Deaminase-Preliminary Experimental and Theoretical Studies

Adenosine deaminase (ADA) is one of the most important enzymes in nucleoside metabolism, regulating the levels of adenosine and deoxyadenosine triphosphate (ADT/dATP) on either side of the cell membrane. This small protein (weighing approximately 40 kDa) exhibits deamination properties towards other pharmaceuticals built on adenine as the leading structure, which requires co-administration of ADA inhibitors. 3'-deoxyadenosine (Cordycepin, Cord) is an active compound isolated from the fungus Cordyceps, which has been used in traditional Chinese medicine for over 2000 years. Its anticancer activity is likely related to the inhibition of primer elongation of lagging strands during genetic information replication. Unfortunately, Cord is rapidly deaminated by ADA into inactive 3'-deoxyinosine, necessitating its co-administration with ADA inhibitors. Here, for the first time, the synthesis and discussion of the oxidised form of Cord are presented. The 7,8-dihydro-8-oxo-3'-deoxyadenosine (Cord^OXO) exhibits high resistance to ADA because of its syn conformation, as shown experimentally by UV spectroscopy and RP-HPLC monitoring. Theoretical Density Functional based Tight Binding (DFTB) studies of the Michaelis complex ADA-Cord^OXO have revealed significant distance increases between the "active" H₂O molecule and C6 of the 8-oxo-adenine moiety of Cord^OXO, i.e., 4 Å as opposed to 2.7 Å in the cases of ADA-dAdo and Cord. In conclusion, it can be postulated that the conversion of Cord to Cord^OXO enhances its therapeutic potential; however, this needs to be verified in vitro and in vivo. It should be emphasised that the therapeutic effect, if any, can be achieved theoretically without ADA inhibitors, e.g., pentostatin, thus reducing adverse effects. These promising preliminary results, presented here, warrant further investigations.