Matched pair suitability of 89Zr and 177Lu-labeled L804-LLP2A theranostics for targeting VLA-4 in melanoma

Abstract

There is a need for chelators that are versatile for complexing multiple radiometals. We previously demonstrated that the macrocyclic 1,2-HOPO chelator, L804, has utility for complexing ¹⁷⁷Lu(III) for radiopharmaceutical therapy and ⁸⁹Zr(IV) for PET imaging with antibody-based agents. Here, we investigated whether ⁸⁹Zr-radiolabelled peptidomimetic LLP2A could function as a diagnostic surrogate for [¹⁷⁷Lu]Lu-LLP2A therapy in the targeting of VLA-4 overexpressed in melanoma, with and without an albumin-binding moiety (pIBA) for biological half-life extension.

Methods: L804-LLP2A (1) and L804-pIBA-LLP2A (2) were synthesized and radiolabelled with ⁸⁹Zr and ¹⁷⁷Lu. Distribution coefficients, in vitro binding affinity, biodistribution, and PET/SPECT imaging data in the B16F10 tumor-bearing mice model of metastatic melanoma were collected.

Results: Quantitative radiochemical yield (>99 %) was achieved with ⁸⁹Zr and ¹⁷⁷Lu within 30 min at 25 °C. Through a shake flask method, the LogD_7.4 distribution coefficients of [⁸⁹Zr]Zr-2 and [¹⁷⁷Lu]Lu-2 (-0.67 ± 0.06 and - 1.05 ± 0.10, respectively) indicated greater lipophilicity compared to their non-albumin binder containing analogues, [⁸⁹Zr]Zr-1 and [¹⁷⁷Lu]Lu-1 (-1.67 ± 0.19 and - 1.26 ± 0.12, respectively). The addition of pIBA to the tracer scaffold did not appreciably affect binding affinity of ⁸⁹Zr- or ¹⁷⁷Lu-labeled LLP2A to VLA-4, with the K_d values for [⁸⁹Zr]Zr-1 vs [⁸⁹Zr]Zr-2 (2.2 ± 0.2 vs 1.6 ± 0.2 nM) and [¹⁷⁷Lu]Lu-1 vs [¹⁷⁷Lu]Lu-2 (8.4 ± 1.0 vs 10.8 ± 2.6 nM) being comparable. In B16F10 melanoma tumor-bearing mice, both [⁸⁹Zr]Zr-1 and [¹⁷⁷Lu]Lu-1 exhibited rapid blood clearance but had distinctly different biodistribution profiles. [⁸⁹Zr]Zr-1 was retained more in the tumor, kidney, and spleen out to 48 h compared to [¹⁷⁷Lu]Lu-1. There was a rapid clearance from both tumor and normal tissues for [¹⁷⁷Lu]Lu-1 out to 48 h, and aside from the muscle, tumor: non-tumor ratios were overall greater for [¹⁷⁷Lu]Lu-1 than those obtained for [⁸⁹Zr]Zr-1. The albumin-binding [⁸⁹Zr]Zr-2 and [¹⁷⁷Lu]Lu-2 displayed more similar distribution profiles, with higher tumor and non-tumor tissue accumulation, and significantly slower blood clearance. Both tracers gradually cleared out of tumor and non-tumor tissues, although [⁸⁹Zr]Zr-2 had slightly more retention in the kidney and tumor at 96 h compared to [¹⁷⁷Lu]Lu-2.

Conclusion: The addition of pIBA to LLP2A resulted in a similar distribution profile between [⁸⁹Zr]Zr-2 and [¹⁷⁷Lu]Lu-2 in contrast to the non-pIBA bearing analogues, where the ⁸⁹Zr and ¹⁷⁷Lu agents showed markedly different biodistributions, indicating the benefit of an albumin binding moiety for a matched small-molecule theranostic pair using these chemically disparate isotopes.

Keywords: Albumin Binder; PET imaging; Radiometal Chelate; Radiopharmaceutical therapy.