Evaluating 225Ac and 177Lu Radioimmunoconjugates against Antibody-Drug Conjugates for Small-Cell Lung Cancer

Interest in the use of Ac-225 for targeted alpha therapies has increased dramatically over the past few years, resulting in a multitude of new isotope production and translational research efforts. However, Ac-225 radioimmunoconjugate (RIC) research is still in its infancy, with most prior experience in hematologic malignancies and only one reported preclinical solid tumor study using Ac-225 RICs. In an effort to compare Ac-225 RICs to other current antibody conjugates, a variety of RICs are tested against intractable small-cell lung cancer (SCLC). We directly compare, in vitro and in vivo, two promising candidates of each alpha or beta(-) category, Ac-225 and Lu-177, versus pyrrolobenzodiazepine (PBD) nonradioactive benchmarks. The monoclonal antibody constructs are targeted to either delta like 3 protein (DLL3), a recently discovered SCLC target, or CD46 as a positive control. An immunocompromised maximum tolerated dose assay is performed on NOD SCID mice, along with tumor efficacy proof-of-concept studies in vivo. We overview the conjugation techniques required to create serum-stable RICs and characterize and compare in vitro cell killing with RICs conjugated to nonspecific antibodies (huIgG1) with either native or site-specific thiol loci against tumor antigen DLL3-expressing and nonexpressing cell lines. Using patient-derived xenografts of SCLC onto NOD SCID mice, solid tumor growth was controlled throughout 3 weeks before growth appeared, in comparison to PBD conjugate controls. NOD SCID mice showed lengthened survival using Ac-225 compared to Lu-177 RICs, and PBD dimers showed full tumor suppression with nine out of ten mice. The exploration of RICs on a variety of antibody-antigen systems is necessary to direct efforts in cancer research toward promising candidates. However, the anti-DLL3-RIC system with Ac-225 and Lu-177 appears to be not as effective as the anti-DLL3-PBD counterpart in SCLC therapy with matched antibodies and portrays the challenges in both SCLC therapy as well as the specialized utility of RICs in cancer treatment.