Development of CD46 targeted alpha theranostics in prostate cancer using ¹³⁴Ce/²²⁵Ac-Macropa-PEG₄-YS5

Abstract

Rationale: 225 Ac, a long-lived α-emitter with a half-life of 9.92 days, has garnered significant attention as a therapeutic radionuclide when coupled with monoclonal antibodies and other targeting vectors. Nevertheless, its clinical utility has been hampered by potential off-target toxicity, a lack of optimized chelators for 225 Ac, and limitations in radiolabeling methods. In a prior study evaluating the effectiveness of CD46-targeted radioimmunotherapy, we found great therapeutic efficacy but also significant toxicity at higher doses. To address these challenges, we have developed a radioimmunoconjugate called 225 Ac-Macropa-PEG 4 -YS5, incorporating a stable PEGylated linker to maximize tumoral uptake and increase tumor-to-background ratios. Our research demonstrates that this conjugate exhibits greater anti-tumor efficacy while minimizing toxicity in prostate cancer 22Rv1 tumors. Methods: We synthesized Macropa.NCS and Macropa-PEG 4/8 -TFP esters and prepared Macropa-PEG 0/4/8 -YS5 (with nearly ~1:1 ratio of macropa chelator to antibody YS5) as well as DOTA-YS5 conjugates. These conjugates were then radiolabeled with 225 Ac in a 2 M NH 4 OAc solution at 30 °C, followed by purification using YM30K centrifugal purification. Subsequently, we conducted biodistribution studies and evaluated antitumor activity in nude mice (nu/nu) bearing prostate 22Rv1 xenografts in both single-dose and fractionated dosing studies. Micro-PET imaging studies were performed with 134 Ce-Macropa-PEG 0/4/8 -YS5 in 22Rv1 xenografts for 7 days. Toxicity studies were also performed in healthy athymic nude mice. Results: As expected, we achieved a >95% radiochemical yield when labeling Macropa-PEG 0/4/8 -YS5 with 225 Ac, regardless of the chelator ratios (ranging from 1 to 7.76 per YS5 antibody). The isolated yield exceeded 60% after purification. Such high conversions were not observed with the DOTA-YS5 conjugate, even at a higher ratio of 8.5 chelators per antibody (RCY of 83%, an isolated yield of 40%). Biodistribution analysis at 7 days post-injection revealed higher tumor uptake for the 225 Ac-Macropa-PEG 4 -YS5 (82.82 ± 38.27 %ID/g) compared to other conjugates, namely 225 Ac-Macropa-PEG 0/8 -YS5 (38.2 ± 14.4/36.39 ± 12.4 %ID/g) and 225 Ac-DOTA-YS5 (29.35 ± 7.76 %ID/g). The PET Imaging of 134 Ce-Macropa-PEG 0/4/8 -YS5 conjugates resulted in a high tumor uptake, and tumor to background ratios. In terms of antitumor activity, 225 Ac-Macropa-PEG 4 -YS5 exhibited a substantial response, leading to prolonged...

Exploring the PET in vivo generator134Ce as a theranostic match for225Ac

“…Unchelated [ 134 Ce]CeCl 3 and [ 225 Ac]AcCl 3 are mostly similar in their biodistribution in mice, with the major difference being the higher spleen uptake found for 134 Ce. Once incorporated in a radiotracer, the radiometals make for a good pharmacokinetic match, as indicated in this study and recent literature [5, 6, 8, 10].…”

“…Hence, chelators with a very restricted coordination environment, like DOTA, DTPA and macropa, are reported to stabilize Ce(III), which then mimics the chemical behaviour of Ac(III), while chelators like 3,4,3-LI(1,2-HOPO) stabilize Ce(IV). In this nature, Ce(IV) has more chemical similarity to tetravalent metals like Th(IV) [5, 8, 10]. Hence, our studies focus on DOTA and macropa-based radiotracers.…”

“…This PET in vivo generator is currently undergoing preclinical investigation in cancer research as a diagnostic match for actinium-225 ( 225 Ac, t ½ =9.9 d) [5][6][7][8]. Over the last decade, 225 Ac has been developed into potent drug constructs targeting various forms of mostly highly aggressive and even treatmentresistant cancers.…”