First-in-class ACP3 high-affinity ligands isolated from DNA-Encoded Chemical Libraries for the delivery of radionuclides against prostate cancer

Introduction: Lutetium Vipivotide Tetraxetan (Pluvicto^TM, a Novartis product), a PSMA-targeted Radio Ligand Therapeutic (RLT), has revolutionized the treatment of metastatic castration-resistant prostate cancer. Despite the clinical benefit observed in large pivotal trials, virtually all patients treated with the drug eventually relapse (VISION trial). Moreover, the product shows an on-target off tumor uptake in healthy tissues, such as salivary glands and kidneys, which is responsible for certain toxicities observed in the clinic. Prostatic Acid Phosphatase (ACP3) is a non-specific phosphomonoesterase synthesized by prostate epithelial cells (Babain et al., Seminars in Nuclear Medicine, 1989, 4:309). ACP3 expression levels are elevated in most prostate cancer lesions, while the antigen is virtually absent in other healthy organs, including salivary glands and kidneys (). Technetium-99m and Indium-111 labeled anti-ACP3 antibody fragments have been successfully used for the radioimmunodetection of metastatic prostate cancer lesions in patients (Vihko et al., Prostate, 1987, 11:51). Here, we present the isolation of first-in-class high-affinity ACP3 ligands from DNA-Encoded Chemical Libraries.

Methods: ACP3 ligands were isolated from DNA-Encoded Chemical Libraries of ~5.8 million members. The most enriched compounds were synthesized as DOTAGA and fluorescein conjugates to measure their affinity against recombinant human ACP3 and binding on antigen-positive cancer cells. Lutetium-177-labeled hit compounds were injected in tumor-bearing mice to assess their in vivo biodistribution and anti-cancer activity.

Results: ProX1, ProX2, and ProX3 were identified as highly potent small organic binders and inhibitors of human ACP3. Affinities in the picomolar range were measured for the three compounds by surface plasmon resonance. The ProX1 ligand (also named “OncoACP3”) resulted in the best tumor accumulation and residence time after intravenous dosing in mice bearing HT-1080.hACP3 or PC3.hACP3 subcutaneous cancer lesions (i.e., >35 %ID/g, 72 hours after administration) with low salivary gland and kidney uptake. Low doses of ¹⁷⁷Lu-OncoACP3 (i.e., 5 and 20 MBq/mouse) induced durable cancer remissions in the HT-1080.hACP3 in vivo tumor model.

Conclusions: Given its exceptional tumor-targeting performance and the lack of salivary glands and kidney uptake, OncoACP3 promises to be the next best-in-class targeting agent for prostate cancer for ligand-based pharmacodelivery applications.