Background:  

High-grade serous (HGS) ovarian adenocarcinoma is characterized by its silent progression and frequent diagnosis at late stages. Early-stage identification and subsequent treatment of ovarian cancer are areas of unmet need, with approximately 13,000 deaths occurring annually in the US alone.¹ Therefore, there is a critical need for additional detection methods and new targeted therapies that can improve patient survival. Human trophoblast cell surface antigen 2 (Trop-2) is an overexpressed glycoprotein in many HGS cancers, presenting a promising diagnostic and therapeutic target. Sacituzumab-Govitcan (SG) is an antibody-drug-conjugate (ADC) targeting Trop-2 with an active cytotoxic warhead (SN-38).  We have developed a radiotheranostic version of SG for immunoPET imaging and are further investigating its potential as a radiopharmaceutical. 

Methods:  

SG was conjugated with DFO via random lysine conjugation and was subsequently radiolabeled with the positron-emitting radionuclide ⁸⁹Zr, yielding a high radiochemical purity (>98%) and a specific activity of 10-15 µCi/µg. Bead binding and cell binding assays confirmed targeting of [⁸⁹Zr]Zr-DFO-SG to Trop-2 positive cells. ImmunoPET and radioimmunotherapy experiments were performed in nude mice bearing Trop-2 positive subcutaneous OVCAR3 xenografts once tumor volume reached about 150-400 mm³. Mice were imaged at 24-, 48-, 72-, and 144- hours post-injection. Utilizing the theranostic approach to treat what we see, we conjugated SG with CHX-A”-DTPA, which was then radiolabeled with ¹⁷⁷Lu to yield a high radiochemical purity (>98%). For our ²²⁵Ac studies, SG was conjugated to TCO, and then clicked to Macropa- PEG₈-Tz in one approach, while in the other, SG was conjugated to tetrafluoropyridyl (TFP) and then to Macropa-PEG₄. We used both macropa derivatives to radiolabel ²²⁵Ac for therapy. Both methods resulted in a quantitative radiochemical purity of (>99%) and a specific activity of 1 µCi/µg. In each of the radiotherapies, cerenkov imaging was conducted 1-4 days after injection of the radiotracer to assess tumor uptake. Mice were monitored twice weekly, with endpoints determined by tumor volume >2,000 mm³, >20% loss in body weight, or the presence of petechia. Moreover, weekly complete blood count (CBC) analysis was conducted to monitor WBC, RBC, and platelet levels.

Results:  

We observed high uptake in [⁸⁹Zr]Zr-DFO-SG tumors, with intensities exceeding 25% injected dose per gram. By 144 hours minimal to no uptake was observed in other key organs, highlighting SG’s potential as a targeting agent. The [¹⁷⁷Lu]Lu-CHX-A’’-DTPA-SG and [²²⁵Ac]Ac-Macropa-PEG₄-TFP-SG significantly suppressed tumor growth in the two cohorts of xenografted OVCAR3s. However, static tumor behavior was observed in the ¹⁷⁷Lu treated mice and by the sixth week tumor recurrence occurred. [⁸⁹Zr]Zr-DFO-SG PET imaging was again performed to confirm OVCAR3 tumors were Trop-2 positive and retreated with 500 μCi of [¹⁷⁷Lu]Lu-CHX-A’’-DTPA-SG. The median survival of [¹⁷⁷Lu]Lu-CHX-A’’-DTPA-SG treated mice was extended to 21.7 weeks compared to 6.1 weeks for SG-only treated mice. In the parallel therapy studies with ²²⁵Ac, we observed a successful reduction of tumor burden in mice; with a notable difference in tumor reduction between TCO and TFP conjugation methods. WBC, RBC, and platelet counts decreased within 2-3 weeks post-injection but returned to normal range by weeks 3-5.  

Conclusion:  

We have built on SG, an FDA-approved treatment, by incorporating a suitable chelator system for non-invasive imaging or therapy. This modification transforms SG into a dual-function radiopharmaceutical for targeting Trop-2-positive ovarian cancers, addressing the need for improved diagnostic tools and therapeutic approaches in HGS ovarian cancers. 

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