A rapid entry to novel and patentable cannabinoid receptor (CB-1) ligands has been identified using computer-based de novo design in combination with parallel synthesis. Small targeted compound arrays were readily prepared from the designs yielding active hit rates of 6% and 10%, respectively. This represents a hit rate enrichment of up to two orders of magnitude compared with corporate compound collections and is the first report of the application of the TOPAS algorithm for the de novo design of GPCR ligands.G-protein coupled receptors (GPCRs) are by far the largest single group of targets within pharmaceutical research today [1]. Not only are they involved in various pharmacological pathways but are also implicated in many different therapeutic indications ranging from infectious and metabolic diseases to CNS disorders. Within the GPCR protein family, extremely limited biostructural information is available due to inherent difficulties arising from large protein size and high lipophilicity. This makes isolation and biostructural characterization of GPCRs a major challenge currently. Rational drug design based upon structural homology is challenging due to the large size and complexity of the target proteins. This requires both computationally intensive and lengthy molecular dynamics refinement as well as detailed mutagenesis efforts to produce an accurate, validated and predictive model due to our limited understanding of ligandreceptor interaction patterns in GPCRs [2]. Successful examples of this approach have been reported ever toward designing novel cannabinoid receptor ligands by structurebased methods which rely on receptor binding-site models [3]. However in general, ligand discovery efforts are commonly initiated based on a high throughput random screening (HTS) program placing pressure on assay development, compound library management and the corresponding logistics, as key bottlenecks. Since the majority of −drugable× targets within pharmacological research belong to this class of proteins it is clear that there is a need for novel technologies which complement classical HTS approaches [4]. Herein we describe the application of an iterative ligand-based library design strategy for the production of small focused compound collections with the goal of generating novel and patentable ligands for the GPCR cannabinoid subtype receptor 1 (CB-1).With the advent of combinatorial chemistry and ultra-HTS, the generation of large compound libraries has become commonplace within the drug discovery industry. Initially, library designs were simply based on chemical feasibility rather than target relevant criteria. Combinatorial chemistry libraries consisted of unpurified mixtures of compounds submitted directly for biological testing, however due to the high false positive and low validated hit rates, the trend is now increasingly towards purified discretes. More recently, a shift from random compound libraries to sophisticated maximally diverse compound collections and targeted compound arrays was favoured [5]. W...