Late-Stage Lead Diversification Coupled with Quantitative Nuclear Magnetic Resonance Spectroscopy to Identify New Structure–Activity Relationship Vectors at Nanomole-Scale Synthesis: Application to Loratadine, a Human Histamine H ₁ Receptor Inverse Agonist

Abstract

An experimental approach is described for late-stage lead diversification of frontrunner drug candidates using nanomole-scale amounts of lead compounds for structure-activity relationship development. The process utilizes C-H bond activation methods to explore chemical space by transforming candidates into newly functionalized leads. A key to success is the utilization of microcryoprobe nuclear magnetic resonance (NMR) spectroscopy, which permits the use of low amounts of lead compounds (1-5 μmol). The approach delivers multiple analogues from a single lead at nanomole-scale amounts as DMSO-<i>d</i>₆ stock solutions with a known structure and concentration for <i>in vitro</i> pharmacology and absorption, distribution, metabolism, and excretion testing. To demonstrate the feasibility of this approach, we have used the antihistamine agent loratadine (<b>1</b>). Twenty-six analogues of loratadine were isolated and fully characterized by NMR. Informative SAR analogues were identified, which display potent affinity for the human histamine H₁ receptor and improved metabolic stability.