Antitumor efficacy of⁹⁰Y-NM600 targeted radionuclide therapy and PD-1 blockade is limited by regulatory T cells in murine prostate tumors

Abstract

BackgroundSystemic radiation treatments that preferentially irradiate cancer cells over normal tissue, known as targeted radionuclide therapy (TRT), have shown significant potential for treating metastatic prostate cancer. Preclinical studies have demonstrated the ability of external beam radiation therapy (EBRT) to sensitize tumors to T cell checkpoint blockade. Combining TRT approaches with immunotherapy may be more feasible than combining with EBRT to treat widely metastatic disease, however the effects of TRT on the prostate tumor microenvironment alone and in combinfation with checkpoint blockade have not yet been studied.MethodsC57BL/6 mice-bearing TRAMP-C1 tumors and FVB/NJ mice-bearing Myc-CaP tumors were treated with a single intravenous administration of either low-dose or high-dose 90Y-NM600 TRT, and with or without anti-PD-1 therapy. Groups of mice were followed for tumor growth while others were used for tissue collection and immunophenotyping of the tumors via flow cytometry.Results90Y-NM600 TRT was safe at doses that elicited a moderate antitumor response. TRT had multiple effects on the tumor microenvironment including increasing CD8 +T cell infiltration, increasing checkpoint molecule expression on CD8 +T cells, and increasing PD-L1 expression on myeloid cells. However, PD-1 blockade with TRT treatment did not improve antitumor efficacy. Tregs remained functional up to 1 week following TRT, but CD8 +T cells were not, and the suppressive function of Tregs increased when anti-PD-1 was present in in vitro studies. The combination of anti-PD-1 and TRT was only effective in vivo when Tregs were depleted.ConclusionsOur data suggest that the combination of 90Y-NM600 TRT and PD-1 blockade therapy is ineffective in these prostate cancer models due to the activating effect of anti-PD-1 on Tregs. This finding underscores the importance of thorough understanding of the effects of TRT and immunotherapy combinations on the tumor immune microenvironment prior to clinical investigation.

Profound immunomodulatory effects of 225Ac-NM600 drive enhanced anti-tumor response in prostate cancer

“…Surprisingly, 225 Ac-NM600 combined with anti-PD1 had no effects on anti-tumor responses compared to 225 Ac-NM600 + isotype control or 225 Ac-NM600 alone, most likely because CD8+ cells were also depleted in that group. These results are in accordance with our recently published studies demonstrating that 90 Y-NM600 RPT with anti-PD-1 ICI had no benefits over RPT alone in these models unless Tregs were simultaneously depleted ( 20 ). While these preliminary results showed that, under the tested conditions, the addition of anti-PD1 to 225 Ac-NM600 did not enhance therapeutic effects, further optimization of this combined regimen may uncover synergies between the two therapeutic modalities.…”

“…Serial histological examination of potential organs at risk of radiotoxicity, such as the heart, liver, spleen, and kidney, revealed no signs of acute cellular toxicity. In addition, via CBC analysis, we identified mild bone marrow toxicity with dose-dependent cytopenia on early time points which were confirmed through histology of bone marrow; however, these were resolved by day 28, similar to our previous findings ( 20–22 ). Functional kidneys and liver metabolic assays further confirmed 177 Lu-NM600 and 225 Ac-NM600’s tolerability, with no clinically relevant effects on blood chemistries at any of the IA or time points investigated.…”

“…Tumors were chopped and digested at 37° in RPMI1640 mixed with DNAse and protease inhibitor. After Fc blocking (BD, Franklin Lakes, NJ, 553142) for 20 minutes at 4°C, cells were then stained for CD11b, CD25, GR-1, CD3, MHCII, CD45, CD4, CD8, CD44, KLRG-1-PE, CD69, CD62l, CD103, CD27, and Foxp3 according to previously published protocol ( 20 ). A Thermo Fisher Attune NxT cytometer and FlowJo v10 were used for analysis.…”

“…Our group has exploited the capacity of cancer cells to selectively sequester and retain metabolically resistant phospholipid (19) to develop a series of alkylphosphocholine (APC) chelate analogs targeting a broad spectrum of malignancies, including prostate cancer (20). NM600, an APC analog featuring a DOTA chelating moiety, shows favorable tumor targeting and pharmacokinetics (21)(22)(23) and can be labeled with various radiometals for imaging and RPT, including 86 Y, 90 Y, 177 Lu, 64 Cu, and 225 Ac.…”

“…Notably, these effects manifested at relatively low absorbed radiation doses that did not elicit significant therapeutic responses. Interestingly, in a recent study ( 20 ), we demonstrated that 90 Y-NM600 treatment led to the enrichment of immunosuppressive regulatory T cells (T reg ) in the TME and poor efficacy in combination with anti-PD1 immune-check point inhibition in Tramp-C1 and MyC-CaP murine models of prostate cancer.…”