An Intrapatient Dosimetry Comparison of 177Lu-rhPSMA-10.1 and 177Lu-PSMA-I&T in Patients with Metastatic Castration-Resistant Prostate Cancer

Organ and tumour dosimetry of 177Lu-rhPSMA-10.1, a novel PSMA-targeted therapy: results from a Phase I trial

PURPOSE

To evaluate tumour and normal organ dosimetry of PSMA-targeted RLT 177Lu-rhPSMA-10.1.

METHODS

PSMA-positive mCRPC patients experiencing disease progression following standard-of-care treatment were enrolled and underwent ≤ 3 cycles of 5.55 or 7.40 GBq 177Lu-rhPSMA-10.1 at 6-week intervals. Multi-bed SPECT/CT was conducted 3-, 24-, 48-, and 168-hours post-administration to calculate tumour and organ absorbed doses. Two methods (activity- and anatomy-based) were used for selecting and delineating tumours for dosimetry. Venous blood was collected for radioactivity measurement 30 min before 177Lu-rhPSMA-10.1 administration, and 0.5-, 1.5-, 4-, 24-, and 48-hours post-administration.

RESULTS

Thirteen patients were enrolled; three received 5.55 GBq/cycle and 10 received 7.40 GBq/cycle. Mean absorbed doses were 0.266, 0.130 and 8.87 Gy/GBq in kidneys, salivary glands, and tumours (activity-method), respectively, giving mean tumour-to-kidney and tumour-to-salivary ratios of 32.1 and 73.2, respectively. Tumour dose estimates were consistently higher with the activity-method vs. anatomy-method. Tumour absorbed doses decreased each cycle; Cycle 2 and 3 doses were ~ 37% and ~ 56% lower than Cycle 1 estimates, respectively. 177Lu-rhPSMA-10.1 was rapidly cleared from the blood (effective half-life, 2.2 h). Imaging data showed mean effective half-lives to be 91.4, 33.7 and 45.4 h in tumours, kidneys, and salivary glands, respectively.

CONCLUSION

177Lu-rhPSMA-10.1 delivers high radiation doses to tumours vs. normal organs, facilitated by its favourable pharmacokinetics. Cumulative doses to normal organs were well within established tolerable limits, suggesting higher cumulative radioactivity could be administered in clinical trials. The observation of decreasing tumour absorbed dose with subsequent cycles also supports the exploration of front-loading radioactivity in Phase II.

Preclinical Evaluation of 177Lu-rhPSMA-10.1, a Radiopharmaceutical for Prostate Cancer: Biodistribution and Therapeutic Efficacy

177Lu-rhPSMA-10.1 is a novel radiohybrid prostate-specific membrane antigen (PSMA)-targeted radiopharmaceutical therapy for prostate cancer. We conducted preclinical analyses on non-tumor-bearing BALB/c mice and on prostate cancer human xenograft mouse models (LNCAP and 22Rv1 xenografts) to evaluate its biodistribution and therapeutic efficacy. Methods: Longitudinal biodistribution of 177Lu-rhPSMA-10.1 was evaluated in BALB/c mice and 22Rv1 xenografts. Tissues of interest were harvested, and radioactivity was measured 1-168 h after injection of 1 MBq of 177Lu-rhPSMA-10.1 (4 per time point). Longitudinal biodistribution was compared with 177Lu-PSMA-I&T (1 MBq) in BALB/c mice and at a single time point (15 h) in 22Rv1 xenografts. The therapeutic efficacy of a single administration of 15, 30, or 45 MBq of 177Lu-rhPSMA-10.1 in LNCaP xenografts and 30 MBq of 177Lu-rhPSMA-10.1, 177Lu-PSMA-617, or 177Lu-PSMA-I&T in 22Rv1 xenografts (8 per group) was evaluated. Efficacy versus vehicle was evaluated on the basis of relative tumor volume and survival up to 49 d after administration. Statistical significance was evaluated with t testing (biodistribution data), 2-way repeated-measures ANOVA (tumor volume [analyzed until 3 per group remained]), or Kaplan-Meier log-rank analyses (survival). Results: Biodistribution of 177Lu-rhPSMA-10.1 in the BALB/c and 22Rv1 xenografts showed rapid clearance from blood and other normal tissues within 48 h, with the kidney showing the highest normal-organ uptake. Kidney uptake and retention were lower for 177Lu-rhPSMA-10.1 than for 177Lu-PSMA-I&T (6.5-fold lower at 12 h in BALB/c mice and 6.4-fold lower at 15 h in 22Rv1 xenografts; P < 0.01). High and sustained 177Lu-rhPSMA-10.1 tumor uptake was observed in 22Rv1 xenografts. This uptake was 2.3-fold higher than that of 177Lu-PSMA-I&T (15 h; P < 0.05). When efficacy was evaluated, 177Lu-rhPSMA-10.1 significantly suppressed tumor growth versus vehicle from day 11 (P < 0.05) in LNCaP xenografts in a dose-dependent manner and from day 18 (P < 0.05) in 22Rv1 xenografts and significantly prolonged median survival versus vehicle in both models. In 22Rv1 xenografts, 177Lu-rhPSMA-10.1 suppressed tumor growth versus vehicle to a greater extent than did 177Lu-PSMA-I&T (significant growth inhibition from day 25 [P < 0.05]) and similarly in extent to 177Lu-PSMA-617 (from day 18 [P < 0.05]). Overall, compared with 177Lu-PSMA-I&T, 177Lu-rhPSMA-10.1 suppressed tumor growth for longer than 177Lu-PSMA-617 (inhibition from day 39 onward [P < 0.05] versus on day 49 only [P < 0.05]). Conclusion: In preclinical models, 177Lu-rhPSMA-10.1 shows a favorable tumor-to-kidney uptake ratio, and significant antitumor effects, indicating it to be a promising next-generation radiopharmaceutical therapy.

Human-Artificial Intelligence Symbiotic Reporting for Theranostic Cancer Care

Reporting of diagnostic nuclear images in clinical cancer management is generally qualitative. Theranostic treatment with 177Lu radioligands for prostate cancer and neuroendocrine tumors is routinely given as the same arbitrary fixed administered activity to every patient. Nuclear oncology, as currently practiced with 177Lu-prostate-specific membrane antigen and 177Lu peptide receptor radionuclide therapy, cannot, therefore, be characterized as personalized precision medicine. The evolution of artificial intelligence (AI) could change this "one-size-fits-all" approach to theranostics, through development of a symbiotic relationship with physicians. Combining quantitative data collection, collation, and analytic computing power of AI algorithms with the clinical expertise, empathy, and personal care of patients by their physician envisions a new paradigm in theranostic reporting for molecular imaging and radioligand treatment of cancer. Human-AI interaction will facilitate the compilation of a comprehensive, integrated nuclear medicine report. This holistic report would incorporate radiomics to quantitatively analyze diagnostic digital imaging and prospectively calculate the radiation absorbed dose to tumor and critical normal organs. The therapy activity could then be accurately prescribed to deliver a preordained, effective, tumoricidal radiation absorbed dose to tumor, while minimizing toxicity in the particular patient. Post-therapy quantitative imaging would then validate the actual dose delivered and sequential pre- and post-treatment dosimetry each cycle would allow individual dose prescription and monitoring over the entire course of theranostic treatment. Furthermore, the nuclear medicine report would use AI analysis to predict likely clinical outcome, predicated upon AI definition of tumor molecular biology, pathology, and genomics, correlated with clinical history and laboratory data. Such synergistic comprehensive reporting will enable self-assurance of the nuclear physician who will necessarily be deemed personally responsible and accountable for the theranostic clinical outcome. Paradoxically, AI may thus be expected to enhance the practice of phronesis by the nuclear physician and foster a truly empathic trusting relationship with the cancer patient.

Experience of rescue therapy with [177Lu]Lu-rhPSMA-10.1 in patients with primary or acquired resistance to [177Lu]Lu-PSMA-I&T

PURPOSE

Radioligand therapy is an increasingly important option for the treatment of metastatic castrate-resistant prostate cancer (mCRPC). Radiohybrid ligands targeting prostate-specific membrane antigen (PSMA) are a novel group of theranostic radioligand therapy agents for which higher tumour absorbed radiation doses have been demonstrated compared to established PSMA ligands. Here, we report data from ten patients who were treated within a compassionate use program with the radiohybrid PSMA-ligand [177Lu]Lu-rhPSMA-10.1 after experiencing disease progression under treatment with [177Lu]Lu-PSMA-I&T.

METHODS

Ten patients with advanced PSMA-positive prostate cancer who showed progression under treatment with [177Lu]Lu-PSMA-I&T received up to three cycles of rescue therapy with [177Lu]Lu-rhPSMA-10.1 (7.4-8.1 GBq per cycle). Efficacy (PSA response according to PCWG3 and RECIP) and overall survival were evaluated. Adverse events were recorded from first application.

RESULTS

Despite progression with [177Lu]Lu-PSMA-I&T, after the first cycle of [177Lu]Lu-rhPSMA-10.1 rescue therapy, five patients (50%) showed a decrease in serum PSA level. In imaging, three of the ten patients (30%) showed a partial radiologic response. Four of the five patients with a decrease of serum PSA under [177Lu]Lu-rhPSMA-10.1 had initially responded to treatment with [177Lu]Lu-PSMA-I&T but had become resistant. However, the remaining patient had shown continuous disease progression during [177Lu]Lu-PSMA-I&T therapy but showed an immediate response to [177Lu]Lu-rhPSMA-10.1. The additional treatment with [177Lu]Lu-rhPSMA-10.1 was generally well tolerated by all patients.

CONCLUSIONS

Patients showing tumour progression while receiving [177Lu]Lu-PSMA-I&T radioligand therapy may benefit from rescue therapy with the novel radiohybrid PSMA ligand, [177Lu]Lu-rhPSMA-10.1. Higher tumour absorbed radiation doses with [177Lu]Lu-rhPSMA-10.1 may overcome primary and acquired radiation resistance.

Theranostics: Timing is Everything

On stage, and in real life, timing is critical for success. Theranostic cancer care epitomizes the central role of timing in the evolution of efficacious molecular targeted radioligand therapy and its incorporation into routine clinical practice of oncology. Nuclear medicine has returned to its therapeutic roots, having been founded as a medical specialty, over three-quarters of a century ago, with radioiodine therapy of thyroid cancer. The very recent oncologist acceptance of 68Ga/177Lu/225Ac-PSMA effectiveness in treating prostate cancer has re-established the role of the physician in nuclear medicine. This article addresses various important issues in respect of timing related to this resurgence. Training of the required new workforce in technical -omics expertise and physicianly virtues is an urgent priority. Precision in radioligand therapy requires definition of individual radiation absorbed dose (Gy) to tumor and to critical normal organs, preferably prospectively. It is time to abandon one-size-fits-all administration of fixed activities (GBq) in arbitrary cycle intervals and duration. The time has also come to design combination sequenced theranostic-immuno-chemotherapeutic approaches to metastatic cancer to address unmet needs, particularly in pancreatic carcinoma; exploiting the potential of new fibroblast activation protein inhibitor radioligands targeting the tumor microenvironment. Public perception of all things "nuclear," including nuclear medicine, has recently recovered from the general opprobrium and radiophobia of the last half-century. Nuclear is the new green. At last, there have arisen propitious circumstances for the future development of theranostics: The timing is right, now.

Developmental Therapeutics in Metastatic Prostate Cancer: New Targets and New Strategies

There is an unmet need to develop new treatments for metastatic prostate cancer. With the development of targeted radioligand therapies, bispecific T cell engagers, antibody-drug conjugates and chimeric antigen receptor T cell (CAR T) therapies, tumor-associated cell surface antigens have emerged as new therapeutic targets in metastatic prostate cancer. Ongoing and completed clinical trials targeting prostate-specific membrane antigen (PSMA), six transmembrane epithelial antigens of the prostate 1 (STEAP1), kallikrein-related peptidase 2 (KLK2), prostate stem cell antigen (PSCA), and delta-like protein 3 (DLL3) in metastatic prostate cancer were reviewed. Strategies for sequential or combinational therapy were discussed.

First Safety and Efficacy Data with the Radiohybrid 177Lu-rhPSMA-10.1 for the Treatment of Metastatic Prostate Cancer

We recently published the first dosimetry data, to our knowledge, for the radioligand therapy agent 177Lu-rhPSMA-10.1, providing an intrapatient comparison with 177Lu-PSMA-I&T in patients with metastatic prostate cancer. Here, we report efficacy and safety findings from these patients. Methods: Four consecutive patients with prostate-specific membrane antigen (PSMA)-positive metastatic prostate cancer received up to 6 cycles of 177Lu-rhPSMA-10.1 (7.4-7.7 GBq per cycle). Efficacy (prostate-specific antigen response according to Prostate Cancer Working Group 3 criteria and the Response Evaluation Criteria in PSMA PET/CT), progression-free survival, and overall survival were evaluated. Adverse events were recorded from the first dose until 16-24 mo after treatment. Results: The patients received a total activity of 29.6-59.4 GBq (4-6 cycles). Prostate-specific antigen was reduced by 100%, 99%, 88%, and 35%. Progression-free survival was not reached for 2 patients at 24 and 18 mo of follow-up and was 15 and 12 mo for the other 2 patients. One patient had a sustained complete response with 2 y of follow up. All patients were alive at the last time point of data collection. No serious adverse events were reported. Conclusion: 177Lu-rhPSMA-10.1 demonstrated encouraging preliminary efficacy and was well tolerated. Formal clinical trials are now under way to evaluate its potential prospectively (NCT05413850).