A non-high throughput screening (HTS) scaffold-hop program identified the probe candidate ML301 and associated analogs. ML301 met probe nomination criteria, exhibiting full agonist behavior (79 – 93%) with an half maximal effective concentration (EC50) of 2.0 – 4.1 μM against neurotensin 1 receptor (NTR1) in the primary assay. This probe satisfied our secondary assay requirements; namely, the Ca2+ mobilization Fluorescence Imaging Plate Reader (FLIPR) assay (93% efficacy at 298 nM) and good selectivity relative to NTR2 and G protein-coupled receptor (GPR)35. In further profiling, ML301 showed low potential for promiscuity and improved pharmacological data when compared to existing art. A scaffold hop program was initiated in parallel with the originally planned high throughput screening based approach. This approach identified new scaffolds of which the most promising was a quinazoline derivative represented by the singleton MLS-0233108. Medicinal chemistry optimization of MLS-0233108 led to ML314, the most potent molecule in this second series that exhibited full agonist behavior (100 %) on NTR1 (EC50 = 1.9 μM). ML314 showed good selectivity against NTR2 and GPR35, but did not stimulate Ca2+ mobilization. ML314 is potentially a biased agonist operating via the β-arrestin pathway rather than the traditional Gq coupled pathway. Signaling mediated by β-arrestin has distinct biochemical and functional consequences that may lead to physiological advantages as described below. This probe report describes the discovery and properties of ML301 and summarizes the HTS and follow-up campaign, which identified ML314.