Preclinical Development of PNT6555, a Boronic Acid-Based, Fibroblast Activation Protein-α (FAP)-Targeted Radiotheranostic for Imaging and Treatment of FAP-Positive Tumors

The overexpression of fibroblast activation protein-α (FAP) in solid cancers relative to levels in normal tissues has led to its recognition as a target for delivering agents directly to tumors. Radiolabeled quinoline-based FAP ligands have established clinical feasibility for tumor imaging, but their therapeutic potential is limited due to suboptimal tumor retention, which has prompted the search for alternative pharmacophores. One such pharmacophore is the boronic acid derivative N-(pyridine-4-carbonyl)-d-Ala-boroPro, a potent and selective FAP inhibitor (FAPI). In this study, the diagnostic and therapeutic (theranostic) potential of N-(pyridine-4-carbonyl)-d-Ala-boroPro-based metal-chelating DOTA-FAPIs was evaluated. Methods: Three DOTA-FAPIs, PNT6555, PNT6952, and PNT6522, were synthesized and characterized with respect to potency and selectivity toward soluble and cell membrane FAP; cellular uptake of the Lu-chelated analogs; biodistribution and pharmacokinetics in mice xenografted with human embryonic kidney cell-derived tumors expressing mouse FAP; the diagnostic potential of 68Ga-chelated DOTA-FAPIs by direct organ assay and small-animal PET; the antitumor activity of 177Lu-, 225Ac-, or 161Tb-chelated analogs using human embryonic kidney cell-derived tumors expressing mouse FAP; and the tumor-selective delivery of 177Lu-chelated DOTA-FAPIs via direct organ assay and SPECT. Results: DOTA-FAPIs and their natGa and natLu chelates exhibited potent inhibition of human and mouse sources of FAP and greatly reduced activity toward closely related prolyl endopeptidase and dipeptidyl peptidase 4. 68Ga-PNT6555 and 68Ga-PNT6952 showed rapid renal clearance and continuous accumulation in tumors, resulting in tumor-selective exposure at 60 min after administration. 177Lu-PNT6555 was distinguished from 177Lu-PNT6952 and 177Lu-PNT6522 by significantly higher tumor accumulation over 168 h. In therapeutic studies, all 3 177Lu-DOTA-FAPIs exhibited significant antitumor activity at well-tolerated doses, with 177Lu-PNT6555 producing the greatest tumor growth delay and animal survival. 225Ac-PNT6555 and 161Tb-PNT6555 were similarly efficacious, producing 80% and 100% survival at optimal doses, respectively. Conclusion: PNT6555 has potential for clinical translation as a theranostic agent in FAP-positive cancer.

Abstract 3303: Pre-clinical characterization of the novel FAP targeting ligand PNT6555 for imaging and therapy of cancer

Background: Fibroblast Activation Protein-α (FAP) is a transmembrane glycoprotein highly expressed on activated fibroblasts. It is a constitutively active 170 kDa serine protease and a member of the dipeptide peptidase (DPP) family, sharing ~50% homology with DPPIV. FAP expression is only rarely expressed in normal adult tissues and is overexpressed in many epithelial cancers through upregulation on cancer-associated fibroblasts present in the stroma of various types of tumor. POINT BioPharma is developing PNT6555, which comprises a DOTA chelator linked to a FAP-targeting moiety, for imaging and therapeutic applications.

Methods: PNT6555 and its radiometal chelates were evaluated for potency, selectivity, biodistribution and efficacy using biochemical and cellular assays as well as imaging, biodistribution and efficacy studies in tumor bearing mice.

Results: PNT6555 and its gallium (natGa-PNT6555) and lutetium (natLu-PNT6555) chelates showed potent activity in FAP inhibition assays using human, mouse, and rat sources of FAP. PNT6555, natLu-PNT6555 and natGa-PNT6555 also showed significantly reduced potency when tested against PREP and DPPIV, two closely related homologous proteins. In vivo time-course biodistribution studies (by PET-imaging) with 68Ga-PNT6555 showed rapid clearance of 68Ga-PNT6555 from blood through the kidneys and urinary tract, with rising 68Ga-PNT6555 activity observed in the tumor through 60 minutes. At 60 minutes, the tumor was the only site of significant retained activity (>10 %ID/g). In vivo biodistribution studies (by SPECT imaging and direct organ assay) with 177Lu-PNT6555 showed rapid renal clearance into the bladder. After 24 hours, the tumor was the only tissue with significant activity retention. Direct organ assay showed little 177Lu-PNT6555 accumulation and retention in normal tissues with a high level of tumor retention observed out to 168h (>10 %ID/g). Therapeutic studies, using a single dose of 177Lu-PNT6555 or 225Ac-PNT6555, were completed in pre-clinical mouse models of cancer. In the HEK-mFAP model, significant dose responsive efficacy was observed in mice treated with either 177Lu-PNT6555 or 225Ac-PNT6555, with no apparent weight loss observed at all tested dose levels. Several mice experienced long-term survival >100 days at multiple of the tested dose levels.

Conclusions: PNT6555, and its radiometal chelates, are potent and specific inhibitors of FAP. 68Ga/177Lu-PNT6555 showed rapid and prolonged uptake into FAP expressing tumors with limited uptake or retention observed in normal tissues. 177Lu/225Ac-PNT6555 showed compelling efficacy in pre-clinical tumor models that expressed FAP. Clinical studies with imaging and therapeutic chelates of PNT6555 are warranted.

Citation Format: Robin M. Hallett, Sarah E. Poplawski, Mark H. Dornan, Shin Hye Ahn, Shuang Pan, Wu Wengen, Liu Yuxin, David G. Sanford, Valerie S. Hergott, Quang-De Nguyen, Anthony P. Belanger, Jack H. Lai, William Bachovchin, Joe A. McCann. Pre-clinical characterization of the novel FAP targeting ligand PNT6555 for imaging and therapy of cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3303.