Lipophilicity Determines Route of Clearance of a Melanocortin 1 Receptor Targeted Radiopharmaceutical

546 Objectives: $$table_{DC41E09C-5B53-40FB-B8C5-F2DA933B3139}$$New effective therapies are greatly needed for metastatic uveal melanoma which has a very poor prognosis with a median survival of less than one year. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanoma metastases. Recently we published studies that demonstrated conjugation of a MC1R specific ligand (MC1RL) with high affinity and selectivity for MC1R to the 225Ac-DOTA chelate, for development of a targeted alpha-particle therapy (TAT) compound for metastatic uveal melanoma. Our pre-clinical studies demonstrated low toxicity and significantly prolonged survival and decreased metastasis burden. However, we have observed that the 225Ac-DOTA-MC1RL conjugate clears from circulation primarily through the liver, which differs from our previously reported near-infrared fluorescent dye conjugate (MC1RL-800) that has predominant renal clearance. Since La3+ is a non-radioactive surrogate for 225Ac and MC1RL-800 is less lipophilic than the La-DOTA conjugate (LogD7.4 values of -3.1 and -2.6 respectively), we hypothesize that lipophilicity is a factor that can influence the clearance route of peptide conjugates of this size. To test this hypothesis and in order to potentially optimize the pharmacokinetic properties of the targeted peptide ligand, we generated a set of 5 conjugates with a range of Log D7.4 values by varying the linker chemistry for attachment of DOTA to MC1RL and performed in vivo biodistribution (BD) studies to determine the clearance routes. Methods: DOTA-MC1RL conjugates with different linkers such as glutamic acid (Glu) and lysine (Lys) were synthesized and labeled with La3+ and 225Ac (Table 1). La3+ versions were analyzed by competitive binding assay and Log D7.4 was determined by the shake-flask assay using liquid chromatography-mass spectrometry (LC-MS). Radiochemical yield, purity and specific activity were determined for all 225Ac versions. Following injection into BALB/c mice, BD was determined by ex vivo gamma spectroscopy of the tissues. Results: The La3+ conjugates retained high binding affinity for the MC1 receptor, were soluble in aqueous buffer but had variable lipophilicities (Log D7.4 values). The 225Ac conjugates were all synthesized with similar radiochemical yield (≥95), purity (≥99) and specific activity. Table 1 describes the results of these studies and suggests that in vivo clearance can be modulated using linker chemistry that influences the lipophilicity of the radiopharmaceutical. Conclusions: Conjugates were designed, synthesized and labeled with 225Ac or La3+. The labeled conjugates retained high binding affinity, high labeling quality and exhibited a range of lipophilicities. Results suggest a relationship between lipophilicity and clearance route. Radiopharmaceuticals possessing more negative LogD7.4 values cleared renally while those with more positive values underwent hepatobiliary excretion. This finding illustrates how tuning radiopharmaceutical lipophilicity can effectively modulate the route of excretion to improve safety and efficacy.