Immunological effects of radiopharmaceutical therapy

Enhanced Retention of NTSR1-Targeted Radionuclide Therapeutics via Covalent Inhibitors in Pancreatic, Colorectal, and Prostate Cancer Models

Neurotensin receptor subtype 1 (NTSR1) is overexpressed in numerous cancers. Our laboratory is exploring the utilization of covalent cysteine protease inhibitors (e.g., E-64) to increase tumor retention of targeted radionuclide therapeutics (TRTs) through protein adduct formation. Using this approach, we reported [177Lu]Lu-NA-ET1, an NTSR1-targeted construct. In this work, we continue the exploration of [177Lu]Lu-NA-ET1 in three different NTSR1-positive cancer models. [177Lu]Lu-3BP-227, a clinically investigated NTSR1-targeted construct, was utilized as a comparative benchmark. Both [177Lu]Lu-NA-ET1 and [177Lu]Lu-3BP-227 underwent in vitro investigation, including internalization and autoradiographic sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) studies, in NTSR1-positive AsPC-1, HT-29, and PC-3 cell lines. Biodistribution, human radiation dosimetry, and in vivo autoradiographic SDS-PAGE studies were performed by using the same models. A dose escalation study using 585 MBq (15.8 mCi) of [177Lu]Lu-NA-ET1 was implemented in immunocompetent CF-1 mice. In all three cell lines, [177Lu]Lu-NA-ET1 demonstrated similar cellular uptake profiles relative to those of [177Lu]Lu-3BP-227. Biodistribution studies of [177Lu]Lu-NA-ET1 revealed increased (1.9-4.4-fold) tumor retention and radiation dose delivery relative to the control. Analysis of the in vitro and in vivo cellular and tissue lysates showed protein adducts that ranged from approximately 25-35 kDa, consistent with cysteine cathepsins, the speculative protein binding partner. A total of 585 MBq (15.8 mCi) of [177Lu]Lu-NA-ET1 was administered and found to be well-tolerated. Incorporating the covalent inhibitor in [177Lu]Lu-NA-ET1 resulted in an improved retention and radiation dose delivery profile compared to [177Lu]Lu-3BP-227. Examination of the therapeutic potential of [177Lu]Lu-NA-ET1 and further exploration of the chemical biology of this approach is underway.

Is Extraordinary Response and Long-Term Remission of Metastatic Castration-Resistant Prostate Cancer (mCRPC) After [¹⁷⁷Lu]Lu-PSMA Radioligand Therapy Due to an Immunomodulatory Effect (Radiovaccination)? A Dual Center Experience on Super-Responders

BACKGROUND

Prostate-specific membrane antigen (PSMA)-directed radioligand therapy (PRLT) with Lutetium-177 ([177Lu]Lu-PSMA) is a safe and effective treatment for metastatic castration-resistant prostate cancer (mCRPC). The aim of our study was to evaluate clinical variables of patients with extreme response to PRLT and to assess its immunomodulatory potential.

METHODS

This retrospective study included 36 patients from two centers achieving extreme response after [¹⁷⁷Lu]Lu-PSMA PRLT. The primary outcomes were the duration of maintained response in months (MR) and improvement post-therapy-clinically, serologically, and on molecular (PET/CT) imaging. We examined several variables, including pathology, gene sequencing, baseline PSA, Gleason score, prior therapies, number of PRLT cycles, and pattern of disease, to identify potential factors that may influence the extreme response.

RESULTS

Between 2018 and mid-September 2024, 36 men with mCRPC received a mean of three cycles of [177Lu]Lu-PSMA PRLT. Patients were subgrouped according to clinical variables versus MR. A total of 17 patients had ≥12 months MR (17/36, 47%). The longest duration of MR was 99 months and a mean of 17.44 months (95% CI 10.05-24.84). Previous lines of treatment were evaluated for MR, p = 0.172. Pattern of disease (bone, lymph node, liver, and peritoneal) was evaluated for MR, p = 0.721. The Gleason score was evaluated for MR, p = 0.871. Patients with known BRCA sequencing status (n = 12) were analyzed with mean MR: BRCA1/2 wild-type, 6/12 (50%), 6.67 months; BRCA 1/2 negative, 1/12 (8.33%), 7 months; BRCA germline negative and somatic positive, 1/12 (8.33%), 36 months; BRCA germline negative, somatic negative, 2/12 (16.67%), 27 months; and BRCA 2 positive, 2/12 (16.67%), 43 months.

CONCLUSIONS

We propose there may be intrinsic mechanisms suggesting the immunomodulatory enhancement of ionizing radiation, primarily driving extreme responses.

Localized In Vivo Prodrug Activation Using Radionuclides

Radionuclides used for imaging and therapy can show high molecular specificity in the body with appropriate targeting ligands. We hypothesized that local energy delivered by molecularly targeted radionuclides could chemically activate prodrugs at disease sites while avoiding activation in off-target sites of toxicity. As proof of principle, we tested whether this strategy of radionuclide-induced drug engagement for release (RAiDER) could locally deliver combined radiation and chemotherapy to maximize tumor cytotoxicity while minimizing off-target exposure to activated chemotherapy. Methods: We screened the ability of radionuclides to chemically activate a model radiation-activated prodrug consisting of the microtubule-destabilizing monomethyl auristatin E (MMAE) caged by a radiation-responsive phenyl azide, and we interpreted experimental results using the radiobiology computational simulation suite TOPAS-nBio. RAiDER was evaluated in syngeneic mouse models of cancer using the fibroblast activation protein inhibitor (FAPI) agents [99mTc]Tc-FAPI-34 and [177Lu]Lu-FAPI-04 and the prostate-specific membrane antigen (PSMA) agent [177Lu]Lu-PSMA-617, combined with caged MMAE or caged exatecan. Biodistribution in mice, combined with clinical dosimetry, estimated the relationship between radiopharmaceutical uptake in patients and anticipated concentrations of activated prodrug using RAiDER. Results: RAiDER efficiency varied by 70-fold across radionuclides (99mTc > 111In > 177Lu > 64Cu > 32P > 68Ga > 223Ra > 18F), yielding up to 320 nM prodrug activation/Gy of exposure from 99mTc. Computational simulations implicated low-energy electron-mediated free radical formation as driving prodrug activation. Radionuclide-activated caged MMAE restored the prodrug's ability to destabilize microtubules and increased its cytotoxicity by up to 2,600-fold that of the nonactivated prodrug. Mice treated with [99mTc]Tc-FAPI-34 and caged MMAE accumulated concentrations of activated MMAE that were up to 3,000 times greater in tumors than in other tissues. RAiDER with [99mTc]Tc-FAPI-34 or [177Lu]Lu-FAPI-04 delayed tumor growth, whereas monotherapies did not (P < 0.003). Clinically guided dosimetry suggests sufficient radiation doses can be delivered to activate therapeutically meaningful levels of prodrug. Conclusion: This proof-of-concept study shows that RAiDER is compatible with multiple radionuclides commonly used in nuclear medicine and can potentially improve the efficacy of radiopharmaceutical therapies to treat cancer safely. RAiDER thus shows promise as an effective strategy to treat disseminated malignancies and broadens the capability of radiopharmaceuticals to trigger diverse biologic and therapeutic responses.

Localized in vivo prodrug activation using radionuclides

Radionuclides used for imaging and therapy can show high molecular specificity in the body with appropriate targeting ligands. We hypothesized that local energy delivered by molecularly targeted radionuclides could chemically activate prodrugs at disease sites while avoiding activation in off-target sites of toxicity. As proof-of-principle, we tested whether this strategy of " RA dionuclide i nduced D rug E ngagement for R elease" ( RAiDER ) could locally deliver combined radiation and chemotherapy to maximize tumor cytotoxicity while minimizing exposure to activated chemotherapy in off-target sites.

Methods

We screened the ability of radionuclides to chemically activate a model radiation-activated prodrug consisting of the microtubule destabilizing monomethyl auristatin E caged by a radiation-responsive phenyl azide ("caged-MMAE") and interpreted experimental results using the radiobiology computational simulation suite TOPAS-nBio. RAiDER was evaluated in syngeneic mouse models of cancer using fibroblast activation protein inhibitor (FAPI) agents 99m Tc-FAPI-34 and 177 Lu-FAPI-04, the prostate-specific membrane antigen (PSMA) agent 177 Lu-PSMA-617, combined with caged-MMAE or caged-exatecan. Biodistribution in mice, combined with clinical dosimetry, estimated the relationship between radiopharmaceutical uptake in patients and anticipated concentrations of activated prodrug using RAiDER.

Results

RAiDER efficiency varied by 250-fold across radionuclides ( 99m Tc> 177 Lu> 64 Cu> 68 Ga> 223 Ra> 18 F), yielding up to 1.22µM prodrug activation per Gy of exposure from 99m Tc. Computational simulations implicated low-energy electron-mediated free radical formation as driving prodrug activation. Clinically relevant radionuclide concentrations chemically activated caged-MMAE restored its ability to destabilize microtubules and increased its cytotoxicity by up to 600-fold compared to non-irradiated prodrug. Mice treated with 99m Tc-FAPI-34 and caged-MMAE accumulated up to 3000× greater concentrations of activated MMAE in tumors compared to other tissues. RAiDER with 99m Tc-FAPI-34 or 177 Lu-FAPI-04 delayed tumor growth, while monotherapies did not ( P <0.03). Clinically-guided dosimetry suggests sufficient radiation doses can be delivered to activate therapeutically meaningful levels of prodrug.

Conclusion

This proof-of-concept study shows that RAiDER is compatible with multiple radionuclides commonly used in nuclear medicine and has the potential to improve the efficacy of radiopharmaceutical therapies to treat cancer safely. RAiDER thus shows promise as an effective strategy to treat disseminated malignancies and broadens the capability of radiopharmaceuticals to trigger diverse biological and therapeutic responses.

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