Semi-rational design of (north)-methanocarba nucleosides as dual acting A(1) and A(3) adenosine receptor agonists: novel prototypes for cardioprotection

Abstract

Ring-constrained adenosine analogues have been designed to act as dual agonists at tissue-protective A(1) and A(3) adenosine receptors (ARs). 9-Ribosides transformed into the ring-constrained (N)-methanocarba-2-chloro-5'-uronamides consistently lost affinity at A(1)/A(2A)ARs and gained at A(3)AR. Among 9-riboside derivatives, only N(6)-cyclopentyl and 7-norbornyl moieties were extrapolated for mixed A(1)/A(3) selectivity and rat/human A(3)AR equipotency. Consequently, 2 was balanced in affinity and potency at A(1)/A(3)ARs as envisioned and dramatically protected in an intact heart model of global ischemia and reperfusion.

Figure 1

Correlation of K_i values at human A₁ (■), A_2A (◆), and A₃ (▲) receptors of adenosine derivatives in two structural series were compared. The two series compared are: mono-substituted adenosine (9-riboside) derivatives, and tri-subsituted 2-chloro-(N)-methanocarba derivatives. In each case, pairs of compounds in which both members have the same N⁶-sustitution (as indicated) are correlated. The five (N)-methanocarba analogues depicted in this graph contained N⁶-benzyl and phenylethyl-type groups, however the general effects on affinity at each of the three adenosine receptor subtypes were generalized to design new N⁶-cycloalkyl analogues having desired pharmacological properties. For the N⁶ substitutions shown, there was consistent loss of affinity at A₁ and A_2AARs and the effect at the A₃AR ranged from no change to a 14-fold gain of affinity. The (N)-methanocarba compounds shown here were all selective for the A₃ receptor, while the desired property in the new analogues was balanced affinity at A₁ and A₃ receptors.

Figure 2

Anti-ischemic infarct-reducing effects of adenosine receptor agonists. Murine hearts were excised and subjected to normothermic global ischemia and reperfusion with or without (A) adenosine receptor agonists as described in Methods. The adenosine receptor agonists were: (B) A₃ agonist Cl-IB-MECA, 30 nM; (C) A₁ agonist 5, 100 nM; (D) Mixed A₁/A₃ agonist 2, 30 nM. Agonists were infused for five min till the induction of ischemia. The heart was stained with TTC after 35 min of ischemia and 120 min of reperfusion and the infarcted areas were visualized as TTC-negative (pale, white). The infarct was quantified by morphometry and normalized to the whole heart as % necrosis. Data were representative of 6 (adenosine receptor agonist-treated) and 16 (DMSO/vehicle-treated) mice. A vehicle control not subjected to ischemia showed no pale or TTC-negative area.

Scheme 1

Reagents: a) RNH₂, MeOH, triethylamine; b) CH₃NH₂, MeOH; c) TFA, H₂O, MeOH, 70°C; d) 10% Pd/C/ H₂, MeOH.