Tapas Das

Radioligand–antibody conjugates (RadioBodies), also known as radiolabeled antibodies, represent an emerging class that enables targeted delivery of r

[¹⁷⁷Lu]-Lu-DOTA-Trastuzumab is currently under clinical evaluation for radioimmunotherapy (RIT) for HER2-positive breast cancer. This study aimed to enhance its internalization by modifying Trastuzumab with a nuclear localization signal (NLS: PKKKRKV). The NLS, bearing a terminal azide group, was synthesized via solid-phase peptide synthesis. DOTA, a chelator for ¹⁷⁷Lu, was incorporated at the α-amino group of lysine at the NLS's proline end, allowing UV-vis-based drug-to-antibody ratio (DAR) estimation. Both DOTA-Trastuzumab and DOTA-NLS-Trastuzumab, along with their [¹⁷⁷Lu]-Lu-labeled counterparts, were synthesized. In vitro studies in HER2-positive SK-OV-3 and SK-BR-3 cells showed an enhanced internalization of the NLS-modified conjugate. However, ex vivo and in vivo evaluation in SCID mice revealed lower tumor and nontarget organ accumulation for [¹⁷⁷Lu]-Lu-DOTA-NLS-Trastuzumab compared to [¹⁷⁷Lu]-Lu-DOTA-Trastuzumab, indicating altered biodistribution despite improved cellular uptake.

This review offers a comprehensive and insightful overview of the production of therapeutic radionuclides (³²P, ⁸⁹Sr, ^117mSn, ¹⁵³Sm, ¹⁶⁶Ho, ¹⁸⁶Re, ¹⁸⁸Re, ¹⁷⁷Lu and ²²³Ra), which are essential for developing bone-targeting radiopharmaceuticals. These radionuclides are critical not only in the early stages of radiopharmaceutical development but also in ensuring their continued success, reliability, and progress. The manuscript highlights recent innovations and trends in the field, addressing significant challenges and emerging opportunities. It also examines the regulatory environment and emphasizes the growing role of automation in enhancing the production process.

In this study, a novel antibody-drug conjugate (ADC) consisting of Rituximab and Chlorambucil (Rituximab-CMB) was synthesized. The average number of drug molecules attached per Rituximab molecule was determined using MALDI-TOF mass spectrometry, revealing a range of 4-6 drug molecules per antibody. To further improve the therapeutic potential of the ADC, it was radiolabeled with the therapeutic radionuclide ¹⁷⁷Lu via a DOTA chelator, achieving a final radiochemical purity of over 95%. In vitro assays demonstrated that the Rituximab-CMB conjugate had greater cytotoxicity compared to that of both unconjugated Rituximab and Chlorambucil alone. Moreover, [¹⁷⁷Lu]Lu-labeled-Rituximab-CMB (15.67 MBq/mg) exhibited higher radiotoxicity (37.08 ± 1.40% cell death) compared to [¹⁷⁷Lu]Lu-labeled-Rituximab (83.99 MBq/mg) (25.25 ± 0.8% cell death) when administered at similar radioactivity doses. Ex vivo experiments indicated that coinjecting cold Rituximab with the radiolabeled formulations significantly improved tumor accumulation and reduced nontarget organ uptake. SPECT-CT imaging results supported these findings, further confirming the enhanced tumor-targeting and biodistribution of the radiolabeled ADC.

Radiosynovectomy (RSV) represents an advanced therapeutic modality in nuclear medicine, designed to treat chronic inflammatory joint disorders that are unresponsive to conventional therapies. This targeted approach involves the intra‐articular administration of radioactive microparticles containing a β ⁻ ‐emitting radionuclide, selectively eradicating the inflamed synovial membrane while preserving surrounding tissues. As a minimally invasive, nonsurgical procedure routinely performed in outpatient settings, RSV offers a compelling alternative to more invasive interventions. Over time, RSV has evolved significantly, transitioning from the empirical use of radiocolloids to the development of specialized agents tailored for different joint types. Advancements in this field continue to explore a variety of β ⁻ ‐emitting radionuclides with unique emission characteristics, integrated into novel microparticles to improve both specificity and therapeutic efficacy. The selection of an optimal radionuclide hinges on critical nuclear and chemical properties, ensuring effective binding to microparticles and delivering favorable clinical outcomes. This review examines the evolution of RSV in joint disorder management, detailing its mechanisms of action, key factors influencing radionuclide and microparticle selection, and the methodologies involved in their development and production. Additionally, it provides an overview of commonly used radionuclides and microparticles, evaluating their effectiveness within the ever‐evolving landscape of RSV.

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 in-house synthesized porphyrin derivative, namely, 5-carboxymethyelene-oxyphenyl-10,15,20-tris(4-methoxyphenyl) porphyrin (UTriMA), 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 Lu-177, a well-known therapeutic radionuclide with high radiochemical purity (>95%). During in vitro studies, the [Lu-177]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 [Lu-177]Lu-DOTA-UTriMA-Lys-NLS (9.32 +/- 1.27% IA/g at 24 h p.i.) in the tumorous lesion compared to [Lu-177]Lu-UTriMA-Lys-DOTA (2.3 +/- 0.13% IA/g at 24 h p.i.) and [Lu-177]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 [Lu-177]Lu-DOTA-UTriMA-Lys-NLS at different time points. Tumor regression studies performed with [Lu-177]Lu-DOTA-UTriMA-Lys-NLS in the same animal model with two different doses [250 mu Ci (9.25 MBq) and 500 mu Ci (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.

Trastuzumab is a US‐FDA‐approved humanized monoclonal antibody used for the treatment of human epidermal growth factor receptor 2 (HER2)‐positive breast cancer. The aim of the present work is to optimize a freeze‐dried formulation of DOTA‐Trastuzumab conjugate for the preparation of patient doses of [¹⁷⁷Lu]Lu‐Trastuzumab for radioimmunotherapy of breast cancer. The formulation of [¹⁷⁷Lu]Lu‐Trastuzumab usually takes a long time, and thus, such a process is not suitable for the routine preparation of this agent in hospital radiopharmacies. To circumvent this, a pre‐synthesized DOTA‐Trastuzumab conjugate as a freeze‐dried formulation is proposed. In the present work, DOTA‐Trastuzumab conjugate was subjected to a freeze‐drying process after the addition of optimized amounts of radioprotectant and cryoprotectant. [¹⁷⁷Lu]Lu‐DOTA‐Trastuzumab was prepared by incubating the lyophilized powder of the kit vial with medium‐specific activity ¹⁷⁷LuCl₃. The final radiochemical purity of [¹⁷⁷Lu]Lu‐DOTA‐Trastuzumab, prepared using freeze‐dried kit, was determined to be >95%. To ascertain the reproducibility of the procedure, six consecutive batches of the freeze‐dried formulation were prepared, radiolabeled, and evaluated by carrying out both in vitro and ex vivo studies. The consistency of the results of all the six consecutive batches confirmed the robustness and utility of the in‐house optimized freeze‐dried formulation for the preparation of patient doses of [¹⁷⁷Lu]Lu‐Trastuzumab at hospital radiopharmacies.