Evaluation of¹⁷⁷Lu-PSMA-617 SPECT/CT Quantitation as a Response Biomarker Within a Prospective¹⁷⁷Lu-PSMA-617 and NOX66 Combination Trial (LuPIN)

¹⁷⁷Lu-PSMA-617 is an effective and novel treatment in metastatic castration-resistant prostate cancer (mCRPC). Our ability to assess response rates and therefore efficacy may be improved using predictive tools. This study investigated the predictive value of serial ¹⁷⁷Lu-PSMA-617 SPECT/CT (¹⁷⁷Lu SPECT) imaging in monitoring treatment response. <b>Methods:</b> Fifty-six men with progressive mCRPC previously treated with chemotherapy and novel androgen signaling inhibitor were enrolled into the LuPIN trial and received up to 6 doses of ¹⁷⁷Lu-PSMA-617 and a radiation sensitizer (3-(4-hydroxyphenyl)-2H-1-benzopyran-7-ol [NOX66]). ⁶⁸Ga-PSMA-11 and ¹⁸F-FDG PET/CT were performed at study entry and exit, and ¹⁷⁷Lu SPECT from vertex to mid thighs was performed 24 h after each treatment. SPECT quantitative analysis was undertaken at cycles 1 (baseline) and 3 (week 12) of treatment. <b>Results:</b> Thirty-two of the 56 men had analyzable serial ¹⁷⁷Lu SPECT imaging at both cycle 1 and cycle 3. In this subgroup, median prostate-specific antigen (PSA) progression-free survival (PFS) was 6.3 mo (95% CI, 5–10 mo) and median overall survival was 12.3 mo (95% CI, 12–24 mo). The PSA 50% response rate was 63% (20/32). ¹⁷⁷Lu SPECT total tumor volume (SPECT TTV) was reduced in 68% (22/32; median, −0.20 m³ [95% CI, −1.4 to −0.001]) and increased in 31% (10/32; median, 0.36 [95% CI, 0.1–1.4]). Any increase in SPECT TTV was associated with shorter PSA PFS (hazard ratio, 4.1 [95% CI, 1.5–11.2]; <i>P</i> = 0.006). An increase of 30% or more in SPECT TTV was also associated with a shorter PSA PFS (hazard ratio, 3.3 [95% CI, 1.3–8.6]; <i>P</i> =0.02). Tumoral SUV_max was reduced in 91% (29/32) and SUV_mean in 84% (27/32); neither was associated with PSA PFS or overall survival outcomes. PSA progression by week 12 was also associated with a shorter PSA PFS (hazard ratio, 26.5 [95% CI, 5.4–131]). In the patients with SPECT TTV progression at week 12, 50% (5/10) had no concurrent PSA progression (median PSA PFS, 4.5 mo [95% CI, 2.8–5.6 mo]), and 5 of 10 men had both PSA and SPECT TTV progression at week 12 (median PSA PFS, 2.8 mo [95% CI, 1.8–3.7 mo]). <b>Conclusion:</b> Increasing SPECT TTV on quantitative ¹⁷⁷Lu SPECT predicts a short PFS and may play a future role as an imaging response biomarker.