Synthesis, stereochemistry and SAR of a series of minodronate analogues as RGGT inhibitors

Phosphonocarboxylate (PC) analogues of bisphosphonates are of interest due to their selective inhibition of a key enzyme in the mevalonate pathway, Rab geranylgeranyl transferase (RGGT). The dextrarotatory enantiomer of 2-hydroxy-3-(imidazo[1,2-a]pyridin-3-yl)-2-phosphonopropanoic acid (3-IPEHPC, 1) is the most potent PC-type RGGT inhibitor thus far identified. The absolute configuration of (+)-1 in the active site complex has remained unknown due to difficulties in obtaining RGGT inhibitor complex crystals suitable for X-ray diffraction analysis. However, we have now succeeded in crystallizing (−)-1 and here report its absolute configuration (AC) obtained by X-ray crystallography, thus also defining the AC of (+)-1. An Autodock Vina 1.1 computer modeling study of (+)-1 in the active site of modified RGGT binding GGPP (3DSV) identifies stereochemistry-dependent interactions that could account for the potency of (+)-1 and supports the hypothesis that this type of inhibitor binds at the TAG tunnel, inhibiting the second geranylgeranylation step. We also report a convenient 31P NMR method to determine enantiomeric excess of 1 and its pyridyl analogue 2, using α- and β-cyclodextrins as chiral solvating agents, and describe the synthesis of a small series of 1 α-X (X = H, F, Cl, Br; 7a–d) analogues to assess the contribution of the α-OH group to activity at enzyme and cellular levels. The IC50 of 1 was 5–10× lower than 7a–d, and the LED for inhibition of Rab11 prenylation in vitro was 2–8× lower than for 7a–d. However, in a viability reduction assay with J774 cells, 1 and 7b had similar IC50 values, ∼10× lower than those of 7a and 7c–d.

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► The absolute configuration of (+)-3-IPEHPC ((+)-1) is identified.
► The stereospecificity of (+)-1 binding to RGGTase is explained.
► α-Desoxy and α-halo analogues (7a–d) of (+)-1 are less potent.
► A convenient NMR determination of ee in 1 and 7a–d is described.