Development of new conformationally and topographically constrained p60(c-src) PTK inhibitors. Solution and solid-phase approaches for the synthesis of delta-opioid receptor peptidomimetic ligands

Abstract

Based on the efficient substrate for p60ᶜ⁻ˢʳᶜ protein tyrosine kinase (PTK) YIYGSFK (1) (K(m) = 55 μM) obtained by combinatorial methods, we have designed and synthesized a series of conformationally and topographically constrained substrate-based peptide inhibitors for this enzyme. The inhibitors showed IC₅₀ values in low micromolar range (0.1-3 μM). A "rotamer scan" was performed by introducing four stereoisomers of β-Me(2')Nal in the postulated interaction site of peptide inhibitor (23) Y-c[D-Pen-(2')Nal-GSFC]KR-NH₂ (IC₅₀ = 1.6 μM). We found that the χ¹ space constraints imposed by the specialized amino acids, introduced at position 3 of peptide 23, were not as important as the configuration of the Cᵅ of that residue to recognize the active site of Src and Lck PTK, as reflected on the observed selectivity ratios. Cocrystallization studies between Lck and two of our inhibitors are in progress, in a collaboration with Dr. X. Zhu (Kinetix, Pharmaceuticals, Inc.). The results obtained may serve as the basis for the design of Lck and/or Src inhibitors, either peptide or nonpeptide. SL-3111 is a high affinity (IC₅₀ = 8.4 nM) and selective (μ/δ = 2020) δ-opioid receptor peptidomimetic ligand developed in Dr. Hruby's laboratory, as the result of extensive structure-activity relationship (SAR) studies based on peptide leads. However, bioassays (GPI and MVD) and in-vivo antinociception studies on the racemic mixture and both enantiomers of this compound, have shown particular problems such as low potency and toxicity. We have shown the importance of the piperazine ring in this molecule for binding toward the δ-opioid receptor. Thus, maintaining such scaffold we have studied a series of solution and solid-phase approaches toward the synthesis of SL-3111 analogues, which explore wider functional diversity at this heterocyclic ring. Compounds 64-67 were synthesized by solution methods. Analysis of the biological data and molecular modeling studies of these compounds, revealed an interesting trend in terms of the effects of the substituent at position two of the piperazine scaffold. Three different solid-phase protocols were explored toward the development of a combinatorial library of this type of compounds, which may facilitate future SAR studies.