Evaluation of Radiolabeled Girentuximab In Vitro and In Vivo

Girentuximab (cG250) targets carbonic anhydrase IX (CAIX), a protein which is expressed on the surface of most renal cancer cells (RCCs). cG250 labeled with 177Lu has been used in clinical trials for radioimmunotherapy (RIT) of RCCs. In this work, an extensive characterization of the immunoconjugates allowed optimization of the labeling conditions with 177Lu while maintaining immunoreactivity of cG250, which was then investigated in in vitro and in vivo experiments. cG250 was conjugated with S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA(SCN)) by using incubation times between 30 and 90 min and characterized by mass spectrometry. Immunoconjugates with five to ten DOTA(SCN) molecules per cG250 molecule were obtained. Conjugates with ratios less than six DOTA(SCN)/cG250 had higher in vitro antigen affinity, both pre- and postlabeling with 177Lu. Radiochemical stability increased, in the presence of sodium ascorbate, which prevents radiolysis. The immunoreactivity of the radiolabeled cG250 tested by specific binding to SK-RC-52 cells decreased when the DOTA content per conjugate increased. The in vivo tumor uptake was < 10% ID/g and independent of the total amount of protein in the range between 5 and 100 µg cG250 per animal. Low tumor uptake was found to be due to significant necrotic areas and heterogeneous CAIX expression. In addition, low vascularity indicated relatively poor accessibility of the CAIX target.

Cohort Study of Somatostatin-Based Radiopeptide Therapy With [90Y-DOTA]-TOC Versus [90Y-DOTA]-TOC Plus [177Lu-DOTA]-TOC in Neuroendocrine Cancers

Purpose Radiopeptide therapy is commonly performed with a single radioisotope. We aimed to compare the effectiveness of somatostatin-based radiopeptide therapy with a single versus a combination of radioisotopes. Patients and Methods In a cohort study, patients with metastasized neuroendocrine cancer were treated with repeated cycles of 90yttrium-labeled tetraazacyclododecane-tetraacetic acid modified Tyr-octreotide ([90Y-DOTA]-TOC) or with cycles alternating between [90Y-DOTA]-TOC and 177lutetium-labeled DOTA-TOC ([177Lu-DOTA]-TOC) until tumor progression or permanent toxicity. Multivariable Cox regression and competing risk regression were used to study predictors of survival and renal toxicity in patients completing three or more treatment cycles. Results A total of 486 patients completed three or more treatment cycles; 237 patients received [90Y-DOTA]-TOC and 249 patients received [90Y-DOTA]-TOC + [177Lu-DOTA]-TOC. Patients receiving [90Y-DOTA]-TOC + [177Lu-DOTA]-TOC had a significantly longer survival than patients receiving [90Y-DOTA]-TOC alone (5.51 v 3.96 years; hazard ratio, 0.64; 95% CI, 0.47 to 0.88; P = .006). The rates of severe hematologic toxicities (6.3% v 4.4%; P = .25) and severe renal toxicity (8.9% v 11.2%; P = .47) were comparable in both groups. Conclusion [90Y-DOTA]-TOC + [177Lu-DOTA]-TOC was associated with improved overall survival compared with [90Y-DOTA]-TOC alone in patients completing three or more cycles of treatment. Contrary to the current practice in radiopeptide therapy, our results suggest an advantage of using a combination of radioisotopes.