CONFORMATIONAL-ANALYSIS OF THE LIPOPHILIC ANTIFOLATE TRIMETREXATE

The conformational properties of the lipophilic antifolate trimetrexat e (TMQ) were calculated and compared to the structurally-analogous pro totypical antifolate methotrexate (MTX) using both empirical force-fie ld and AM1 quantum mechanical methods. The conformational preferences of TMQ and MTX are diametrically opposed with respect to the bridge-sy stem set of torsion angles tau(1), tau(2): TMQ prefers gauche, trans w hile MTX prefers similar to trans, gauche. These predictions are consi stent with the observed crystal structures of TMQ (i.e., tau(1) = 79 d egrees, tau(2) = 178 degrees) and of DHFR-bound MTX (i.e., tau(1) = -1 57 degrees, tau(2) = 57 degrees in L. casei). The crystal structure of MTX.4H(2)O deviates from this pattern with tau(1) closer to cis (i.e. , 39 degrees) than the predicted trans, yet this near-cis conformation is driven by intermolecular hydrogen-bonding and electrostatic forces operative in the MTX crystal. As a consequence of these strong interm olecular forces, MTX incurs 1.8 kcal/mole in confromational-strain ene rgy in its crystalline form. In contrast, TMQ experiences virtually no conformational strain in its crystalline form. This disparity is attr ibuted to two distinctions between TMQ and MTX: (i) MTX crystallizes a s a zwitterion while TMQ crystallizes as the free base, and (ii) the h ydrophilic glutamate tail in MTX is replaced by three lipophilic trime thoxy groups in TMQ. The corresponding conformational-strain energy of DHFR-bound MTX is 2.0 kcal/mole while that of DHFR-bound TMQ is only 0.65 kcal/mole based on the assumption that the latter adopts the same bridge conformation as the former. This cost in conformational-strain energy for TMQ and MTX is paid at the expense of their respective fre e energies of binding to DHFR. Consequently, the present study offers the possibility of designing a new class of antifolates which are conf ormationally strain-free when bound to DHFR and thereby more effective as chemotherapeutic agents.