Mechanisms, Challenges, and Opportunities in Combined Radiation and Hormonal Therapies

Myeloid-derived suppressor cells attenuate the antitumor efficacy of radiopharmaceutical therapy using 90Y-NM600 in combination with androgen deprivation therapy in murine prostate tumors

RATIONALE

Androgen deprivation therapy (ADT) is pivotal in treating recurrent prostate cancer and is often combined with external beam radiation therapy (EBRT) for localized disease. However, for metastatic castration-resistant prostate cancer, EBRT is typically only used in the palliative setting, because of the inability to radiate all sites of disease. Systemic radiation treatments that preferentially irradiate cancer cells, known as radiopharmaceutical therapy or targeted radionuclide therapy (TRT), have demonstrable benefits for treating metastatic prostate cancer. Here, we explored the use of a novel TRT, 90Y-NM600, specifically in combination with ADT, in murine prostate tumor models.

METHODS

6-week-old male FVB mice were implanted subcutaneously with Myc-CaP tumor cells and given a single intravenous injection of 90Y-NM600, in combination with ADT (degarelix). The combination and sequence of administration were evaluated for effect on tumor growth and infiltrating immune populations were analyzed by flow cytometry. Sera were assessed to determine treatment effects on cytokine profiles.

RESULTS

ADT delivered prior to TRT (ADT→TRT) resulted in significantly greater antitumor response and overall survival than if delivered after TRT (TRT→ADT). Studies conducted in immunodeficient NRG mice failed to show a difference in treatment sequence, suggesting an immunological mechanism. Myeloid-derived suppressor cells (MDSCs) significantly accumulated in tumors following TRT→ADT treatment and retained immune suppressive function. However, CD4+ and CD8+ T cells with an activated and memory phenotype were more prevalent in the ADT→TRT group. Depletion of Gr1+MDSCs led to greater antitumor response following either treatment sequence. Chemotaxis assays suggested that tumor cells secreted chemokines that recruited MDSCs, notably CXCL1 and CXCL2. The use of a selective CXCR2 antagonist, reparixin, further improved antitumor responses and overall survival when used in tumor-bearing mice treated with TRT→ADT.

CONCLUSION

The combination of ADT and TRT improved antitumor responses in murine models of prostate cancer, however, this was dependent on the order of administration. This was found to be associated with one treatment sequence leading to an increase in infiltrating MDSCs. Combining treatment with a CXCR2 antagonist improved the antitumor effect of this combination, suggesting a possible approach for treating advanced human prostate cancer.

Phase II Randomized Study of Salvage Radiation Therapy Plus Enzalutamide or Placebo for High-Risk Prostate-Specific Antigen Recurrent Prostate Cancer After Radical Prostatectomy: The SALV-ENZA Trial

PURPOSE

We sought to investigate whether enzalutamide (ENZA), without concurrent androgen deprivation therapy, increases freedom from prostate-specific antigen (PSA) progression (FFPP) when combined with salvage radiation therapy (SRT) in men with recurrent prostate cancer after radical prostatectomy (RP).

PATIENTS AND METHODS

Men with biochemically recurrent prostate cancer after RP were enrolled into a randomized, double-blind, phase II, placebo-controlled, multicenter study of SRT plus ENZA or placebo (ClinicalTrials.gov identifier: NCT02203695). Random assignment (1:1) was stratified by center, surgical margin status (R0 v R1), PSA before salvage treatment (PSA ≥ 0.5 v < 0.5 ng/mL), and pathologic Gleason sum (7 v 8-10). Patients were assigned to receive either ENZA 160 mg once daily or matching placebo for 6 months. After 2 months of study drug therapy, external-beam radiation (66.6-70.2 Gy) was administered to the prostate bed (no pelvic nodes). The primary end point was FFPP in the intention-to-treat population. Secondary end points were time to local recurrence within the radiation field, metastasis-free survival, and safety as determined by frequency and severity of adverse events.

RESULTS

Eighty-six (86) patients were randomly assigned, with a median follow-up of 34 (range, 0-52) months. Trial arms were well balanced. The median pre-SRT PSA was 0.3 (range, 0.06-4.6) ng/mL, 56 of 86 patients (65%) had extraprostatic disease (pT3), 39 of 86 (45%) had a Gleason sum of 8-10, and 43 of 86 (50%) had positive surgical margins (R1). FFPP was significantly improved with ENZA versus placebo (hazard ratio [HR], 0.42; 95% CI, 0.19 to 0.92; P = .031), and 2-year FFPP was 84% versus 66%, respectively. Subgroup analyses demonstrated differential benefit of ENZA in men with pT3 (HR, 0.22; 95% CI, 0.07 to 0.69) versus pT2 disease (HR, 1.54; 95% CI, 0.43 to 5.47; Pinteraction = .019) and R1 (HR, 0.14; 95% CI, 0.03 to 0.64) versus R0 disease (HR, 1.00; 95% CI, 0.36 to 2.76; Pinteraction = .023). There were insufficient secondary end point events for analysis. The most common adverse events were grade 1-2 fatigue (65% ENZA v 53% placebo) and urinary frequency (40% ENZA v 49% placebo).

CONCLUSION

SRT plus ENZA monotherapy for 6 months in men with PSA-recurrent high-risk prostate cancer after RP is safe and delays PSA progression relative to SRT alone. The impact of ENZA on distant metastasis or survival is unknown at this time.

Androgen Receptor Gene Pathway Upregulation and Radiation Resistance in Oligometastatic Prostate Cancer

Stereotactic ablative body radiotherapy (SABR) is currently used as a salvage intervention for men with oligometastatic prostate cancer (PC), and increasingly so since the results of the Stereotactic Ablative Body Radiotherapy for the Comprehensive Treatment of Oligometastatic Cancers (SABR-COMET) trial reported a significant improvement in overall survival with SABR. The addition of androgen deprivation therapy (ADT) to localised prostate radiotherapy improves survival as it sensitises PC to radiotherapy-induced cell death. The importance of the androgen receptor (AR) gene pathway in the development of resistance to radiotherapy is well established. In this review paper, we will examine the data to determine how we can overcome the upregulation of the AR pathway and suggest a strategy for improving outcomes in men with oligometastatic hormone-sensitive PC.