Assessing the impact of chelation and conjugation chemistry on the therapy effect of 177Lu-labeled antibody targeting uncomplexed PSA in tumor bearing mice

Objectives: 5A10, an antibody specific for an epitope in the catalytic cleft of prostate specific antigen (PSA), and thus specific for free PSA, has shown promising results for imaging of prostate cancer (Ulmert, Cancer Discovery 2012). Data have shown that increasing the cheater to antibody molar ratio, significantly increases the tumor uptake for ¹¹¹In-labed h5A10 . Furthermore, autoradiography data indicates a more homogenous distribution of radioactivity when increasing the cheater to antibody molar ratio (SNMMI, Tampa 2020). The goal of this study was to further investigate if increasing the chelate (DTPA) ration can improve therapy effect of ¹⁷⁷Lu labeled h5A10.

Methods: The h5A10 (Innovagen AB, Lund, Sweden) was conjugated with DTPA at a molar ratio of 1:1, 1:3, 1:6, and 1:12 antibody to chelate. Surface plasmon resonance was used to study the interaction between the antigen, the antibody and the FcRn. For ¹⁷⁷Lu-labelling, DTPA-h5A10 (30µg) was reconstituted in 0.2 M ammonium acetate buffer pH 5.5 and mixed with approximately 20 MBq of ¹⁷⁷Lu- chloride (specific activity= 0.67 MBq/ug). The ratios 1:3, and 1:12 of radiolabeled immunoconjugate (30µg, 20MBq) was injected into groups of mice (n=8) carrying LNCaP xenografts. Four mice in each group were anesthetized at 48 hours post-injection and SPECT imaging was performed. The mice were weighed and tumor size measured for 25 days. Two groups of control mice (n=8) carrying LNCaP xenografts were i.v. injected with 150 µL of PBS solution or non-specific ¹⁷⁷Lu-IgG (20 MBq, 30 ug).

Results: As seen by SPR the binding to PSA is clearly affected by the chelate to antibody ratio during the conjugation reaction, with lower binding at higher ratios. DTPA conjugated to h5A10 at ratio 1:3 and 1:12 antibody to chelate molar ratio was labeled with ¹⁷⁷Lu with high purities and high yields. Tumor uptake of ¹⁷⁷Lu-DTPA-h5A10 ratio 1:3 was 7.6±1.3 %IA/g and for ratio 1:12 7.4±1.20 %IA/g at 48 hours p.i.. The tumor-to liver ratio was 1.04±0.15 for ratio 1:3 and 1.04±0.07 for ratio 1:12 at 48 hours p.i. There was a significant better therapy effect for ¹⁷⁷Lu-DTPA-5A10 ratio 1:12 compared to ¹⁷⁷Lu-DTPA-5A10 1:3, vehicle and ¹⁷⁷Lu-IgG for the 25 days studied.

Conclusions: The present study indicates optimization possibilities of h5A10 an internalizing antibody targeting secreted free PSA in theranostic applications.