Delta-like ligand 3-targeted radioimmunotherapy for neuroendocrine prostate cancer

Abstract

Neuroendocrine prostate cancer (NEPC) is a lethal subtype of prostate cancer with limited meaningful treatment options. NEPC lesions uniquely express delta-like ligand 3 (DLL3) on their cell surface. Taking advantage of DLL3 overexpression, we developed and evaluated lutetium-177 (¹⁷⁷Lu)-labeled DLL3-targeting antibody SC16 (¹⁷⁷Lu-DTPA-SC16) as a treatment for NEPC. SC16 was functionalized with DTPA-CHX-A" chelator and radiolabeled with ¹⁷⁷Lu to produce ¹⁷⁷Lu-DTPA-SC16. Specificity and selectivity of ¹⁷⁷Lu-DTPA-SC16 were evaluated in vitro and in vivo using NCI-H660 (NEPC, DLL3-positive) and DU145 (adenocarcinoma, DLL3-negative) cells and xenografts. Dose-dependent treatment efficacy and specificity of ¹⁷⁷Lu-DTPA-SC16 radionuclide therapy were evaluated in H660 and DU145 xenograft-bearing mice. Safety of the agent was assessed by monitoring hematologic parameters. ¹⁷⁷Lu-DTPA-SC16 showed high tumor uptake and specificity in H660 xenografts, with minimal uptake in DU145 xenografts. At all three tested doses of ¹⁷⁷Lu-DTPA-SC16 (4.63, 9.25, and 27.75 MBq/mouse), complete responses were observed in H660-bearing mice; 9.25 and 27.75 MBq/mouse doses were curative. Even the lowest tested dose proved curative in five (63%) of eight mice, and recurring tumors could be successfully re-treated at the same dose to achieve complete responses. In DU145 xenografts, ¹⁷⁷Lu-DTPA-SC16 therapy did not inhibit tumor growth. Platelets and hematocrit transiently dropped, reaching nadir at 2 to 3 wk. This was out of range only in the highest-dose cohort and quickly recovered to normal range by week 4. Weight loss was observed only in the highest-dose cohort. Therefore, our data demonstrate that ¹⁷⁷Lu-DTPA-SC16 is a potent and safe radioimmunotherapeutic agent for testing in humans with NEPC.

Keywords: DLL3; lutetium-177; neuroendocrine prostate cancer; radioimmunotherapy.

Fig. 1.

Ex vivo biodistribution of ¹⁷⁷Lu-DTPA-SC16 in subcutaneous xenograft model of NEPC shows high and selective tumor uptake. (A) Select organ biodistribution data at 24, 72, and 120 h postintravenous injection of ¹⁷⁷Lu-DTPA-SC16 in athymic male nude mice bearing subcutaneous H660 tumors (n = 3–4/cohort). Tumor uptake at 72 h could be blocked with a 50-fold excess of unlabeled SC16 antibody (****P < 0.0001). (B) Tumor-to-muscle contrast ratios from the uptake of ¹⁷⁷Lu-DTPA-SC16 in H660-bearing mice. (C) Select organ biodistribution data at 24, 72, and 120 h postintravenous injection of ¹⁷⁷Lu-DTPA-SC16 in athymic male nude mice bearing subcutaneous DU145 tumors (n = 3/cohort). (D) Tumor-to-muscle contrast ratios from the uptake of ¹⁷⁷Lu-DTPA-SC16 in DU145-bearing mice.

Fig. 2.

¹⁷⁷Lu-DTPA-SC16 treatment inhibits tumor growth and extends survival in NEPC-bearing mice. Tumor volume (mm³) growth curves (A) and overall survival (%) (B) of H660-bearing mice after treatment. Significant tumor growth inhibition was observed in all treated mice compared with saline cohort (n = 8/cohort; shading represents 95% confidence interval; *P = 0.0154, ***P = 0.0005, ****P < 0.0001). The red notch at day 64 in the 9.25-MBq ¹⁷⁷Lu-DTPA-SC16 cohort refers to mouse 113, which was euthanized unrelated to therapy.

Fig. 3.

Hematologic toxicity is either absent or transient in ¹⁷⁷Lu-DTPA-SC16–treated mice with NEPC tumors. WBC counts (A), RBC counts (B), and PLT counts (C) in H660-bearing mice that received ¹⁷⁷Lu-DTPA-SC16 treatment. Three of the 20 parameters measured are shown. Gray shading indicates the mean ± 1 SD of values collected from the entire cohort of H660-bearing mice prior to therapy initiation (week 0).

Fig. 4.

Re-treatment with ¹⁷⁷Lu-DTPA-SC16 reduces tumor growth again in recurring NEPC tumors. (A) PET images of ⁸⁹Zr-DFO-SC16 in H660-bearing mice with recurring tumors. PET images were performed at 120 h postinjection of ⁸⁹Zr-DFO-SC16, corresponding to 98 d postfirst therapy injection. The images represent maximum intensity projection (MIP) and transverse planar images. See SI Appendix for details on PET imaging. (B) Individual tumor volume (mm³) growth curves of H660-bearing mice with recurring tumors. Mice were retreated with a second 4.63-MBq ¹⁷⁷Lu-DTPA-SC16 injection 100 d following their first 4.63-MBq ¹⁷⁷Lu-DTPA-SC16 injection at day 0.