Pin1 (protein interacting with NIMA 1) is a peptidyl-prolyl isomerase involved in mitosis. As a potential anti-cancer drug target, Pin1 interacts and regulates the activity of an increasing number of cell cycle enzymes by an unknown mechanism. These cell cycle enzymes include Cdc25, Cdc27, Cyclin D1, Myt1, Wee1, NIMA, Cdc2, Plk1 and c-Myc. Recent research has revealed that Pin1 is overexpressed in a variety of cancer cell lines and Pin1 inhibitors inhibit proliferation activity of several cancer cells overexpressing Pin1. The most potent Pin1 inhibitors identified so far are in the micromolar range and no pharmacophore has been identified.

In order to assist the understanding of the biological function of Pin1 using molecular probes, two amide isosteres of Ser-trans-Pro and Ser-cis-Pro dipeptides were designed and stereoselectively synthesized. The conformationally locked Ser–trans–Pro mimic, Boc-SerΨ[(E)CH=C]Pro–OH, was synthesized through the use of an Ireland-Claisen [3,3]-sigmatropic rearrangement in nine steps with 13% overall yield from a serine derivative. The Ser-cis-Pro mimic, Boc-SerΨ[(Z)CH=C]Pro–OH, was synthesized through the use of a Still-Wittig [2,3]-sigmatropic rearrangement in 11 steps with an overall yield of 20% from the same starting material.

Conformationally locked peptidomimetics, including two exactly matched peptidomimetics, Ac–Phe–Phe–pSer–Ψ(E)CH=C]Pro–Arg–NH2 and Ac–Phe–Phe–pSer–Ψ[(Z)CH=C]Pro–Arg–NH2, were synthesized from these Ser-Pro isosteres using Fmoc SPPS. A protocol for in vitro Pin1 inhibition assay was established for measuring the inhibition constant for these peptidomimetics. A conformationally locked cis peptidomimetic inhibits Pin1 with a K_i of 1.7 μM, 23-fold more potent than its trans counterpart, illustrating the preference of Pin1 for a cis amide bond in its PPIase domain. The A2780 ovarian cancer cell antiproliferation activity of these peptidomimetics parallels their respective Pin1 inhibition data. This research provides a start toward more drug-like Pin1 inhibitor design. Gly–trans–Pro isosteres were synthesized using the Ireland-Claisen route. The construction of a non-peptidic (Z)-alkene library for Pin1 inhibition was attempted using the Ser-cis-Pro mimic, Boc—SerΨ[(Z)CH=C]Pro–OH as the core.