Investigations into the Origin of the Molecular Recognition of Several Adenosine Deaminase Inhibitors

Abstract

Inhibitors of adenosine deaminase (ADA, EC 3.5.4.4) are potential therapeutic agents for the treatment of various health disorders. Several highly potent inhibitors were previously identified, yet they exhibit unacceptable toxicities. We performed a SAR study involving a series of C2 or C8 substituted purine-riboside analogues with a view to discover less potent inhibitors with a lesser toxicity. We found that any substitution at C8 position of nebularine resulted in total loss of activity toward calf intestinal ADA. However, several 2-substituted-adenosine, 8-aza-adenosine, and nebularine analogues exhibited inhibitory activity. Specifically, 2-Cl-purine riboside, 8-aza-2-thiohexyl adenosine, 2-thiohexyl adenosine, and 2-MeS-purine riboside were found to be competitive inhibitors of ADA with K(i) values of 25, 22, 6, and 3 μM, respectively. We concluded that electronic parameters are not major recognition determinants of ADA but rather steric parameters. A C2 substituent which fits ADA hydrophobic pocket and improves H-bonding with the enzyme makes a good inhibitor. In addition, a gg rotamer about C4'-C5' bond is apparently an important recognition determinant.

The loss of a hydrogen bond: Thermodynamic contributions of a non-standard nucleotide

“…Due to its structural similarity to (A), P has been used to probe structures to highlight the effects of base modifications on function. For example, P has been used to investigate the activity of adenosine deaminase enzymes (14,15). A-U pairs in RNA duplexes were replaced with P•U pairs, and the effect on enzyme activity was determined (14–18).…”

“…For example, P has been used to investigate the activity of adenosine deaminase enzymes (14,15). A-U pairs in RNA duplexes were replaced with P•U pairs, and the effect on enzyme activity was determined (14–18). In these studies, both with P and its analogues, the loss of the functional group inhibited the function of enzymes that recognize A (14–18).…”

“…A-U pairs in RNA duplexes were replaced with P•U pairs, and the effect on enzyme activity was determined (14–18). In these studies, both with P and its analogues, the loss of the functional group inhibited the function of enzymes that recognize A (14–18). This inhibitory effect could be the result of the enzyme requiring the exocyclic amino group for function or could be the result of decreased stability in the target RNAs due to the A to P replacement.…”

Synthesis of New Nebularine Analogues and Their Inhibitory Activity against Adenosine Deaminase

“…mp: 300-301°C (EtOH). (21) The N-oxide 20 (2.6 g, 10 mmol) was added in portions into fuming nitric acid (13 mL) at 0°C and the mixture was heated at 55°C for 5 h. Upon cooling, the solution was poured into ice (40 mL) and the pH was adjusted to 3 by dropwise addition of ammonium hydroxide 25%. The precipitate was filtered, washed with cold water and vacuum-dried to provide the nitro-compound 21 (1.6 g, 52%) as a yellow solid.…”

“…In recent years research efforts have mainly focused on the investigation of ADA-ligand interactions aiming at the design of less toxic inhibitors of the enzyme. 20,21) Based on the interesting ADA inhibitory activity of several 2-substituted-adenosine and nebularine derivatives 21) and as a part of our ongoing research project directed towards the preparation of novel purine ribosides, [22][23][24][25][26] we were prompted to synthesize a number of new nebularine analogues and evaluate their potential ADA inhibitory activity. Thus, we present here the synthesis and biological evaluation of new 1-deazanebularine analogues, having in mind that the isosteric replacement of a heterocyclic nitrogen by a carbon unit is a well-established procedure for the study of the interactions of purines with their biological targets.…”