Nuclear Localization Signal Enhances the Targeting and Therapeutic Efficacy of a Porphyrin-Based Molecular Cargo: A Systemic In Vitro and Ex Vivo Evaluation

Abstract

The objective of the present work was to evaluate the potential of a nuclear localization signal (NLS) toward facilitating intracellular delivery and enhancement in the therapeutic efficacy of the molecular cargo. Toward this, an inhouse synthesized porphyrin derivative, namely, 5-carboxymethyelene-oxyphenyl-10,15,20-tris(4-methoxyphenyl) porphyrin (UTri-MA), was utilized for conjugation with the NLS sequence [PKKKRKV]. The three compounds synthesized during the course of the present work, namely DOTA-Lys-NLS, DOTA-UTriMA-Lys-NLS, and DOTA-Lys-UTriMA, were evaluated for cellular toxicity in cancer cell lines (HT1080), wherein all exhibited minimal dark toxicity. However, during photocytotoxicity studies with DOTA-Lys-UTriMA and DOTA-UTriMA-Lys-NLS conjugates in the same cell line, the latter exhibited significantly higher light-dependent toxicity compared to the former. Furthermore, the photocytotoxicity for DOTA-UTriMA-Lys-NLS in a healthy cell line (WI26VA4) was found to be significantly lower than that observed in the cancer cells. Fluorescence cell imaging studies carried out in HT1080 cancer cells revealed intracellular accumulation for the NLS-conjugated porphyrin (DOTA-UTriMA-Lys-NLS), whereas unconjugated porphyrin (DOTA-Lys-UTriMA) failed to do so. To evaluate the radiotherapeutic effects of the synthesized conjugates, all three compounds were radiolabeled with 177 Lu, a well-known therapeutic radionuclide with high radiochemical purity (>95%). During in vitro studies, the [ 177 Lu]Lu-DOTA-UTriMA-Lys-NLS complex exhibited the highest cell binding as well as internalization among the three radiolabeled complexes. Biological distribution studies for the radiolabeled compounds were performed in a fibrosarcoma-bearing small animal model, wherein significantly higher accumulation and prolonged retention of [ 177 Lu]Lu-DOTA-UTriMA-Lys-NLS (9.32 ± 1.27% IA/g at 24 h p.i.) in the tumorous lesion compared to [ 177 Lu]Lu-UTriMA-Lys-DOTA (2.3 ± 0.13% IA/g at 24 h p.i.) and [ 177 Lu]Lu-DOTA-Lys-NLS complexes (0.26 ± 0.17% IA/g at 24 h p.i.) were observed. The results of the biodistribution studies were further corroborated by recording serial SPECT-CT images of fibrosarcoma-bearing Swiss mice administered with [ 177 Lu]Lu-DOTA-UTriMA-Lys-NLS at different time points. Tumor regression studies performed with [ 177 Lu]Lu-DOTA-UTriMA-Lys-NLS in the same animal model with two different doses [250 μCi (9.25 MBq) and 500 μCi (18.5 MBq)] resulted in a significant reduction in tumor mass in the treated group of animals. The above results revealed a definite enhancement in the targeting ability of molecular cargo upon conjugation with NLS and hence indicated that this strategy may be helpful for the preparation of drug-NLS conjugates as multimodal agents.