Eligibility for 177Lu-PSMA therapy depends on the choice of companion diagnostic tracer: a comparison of 68Ga-PSMA-11 and 99mTc-MIP-1404 in metastatic castrate resistant prostate cancer.

Lutetium-177-prostate-specific membrane-617 (177Lu-PSMA-617) is an effective therapy for metastatic castration resistant prostate cancer (mCRPC) with evidence of improved survival over standard care. The VISION trial inclusion criteria required a metastatic lesion to liver ratio of > 1 on 68Ga-PSMA-11 positron emission tomography (PET) scans. We aimed to determine if an equivalent ratio is suitable for a single-photon emission computed tomography (SPECT) tracer, 99mTc-MIP-1404, and to compare lesion and lesion to normal organ ratios between the two radiotracers. Methods: Two cohorts of patients with mCRPC matched for age, prostate-specific antigen (PSA) level and total Gleason score, with either 99mTc-MIP-1404 SPECT/CT (n = 25) or 68Ga-PSMA-11 PET/CT (n = 25) scans, were included for analysis. Up to 3 lesions in each site (prostate/prostate bed, lymph nodes, bone and soft tissue metastases) as well as normal liver, parotid gland, spleen and mediastinal blood pool maximum standardized uptake values (SUVmax) were measured. Results: 99mTc-MIP-1404 SPECT lesion SUVmax was not significantly different from 68Ga-PSMA-11 PET (median 18.2 vs 17.3; P = 0.93). However, 99mTc-MIP-1404 liver SUVmax was higher (median 8.5 vs 5.8; P = 0.002) and lesion to liver ratios were lower (median 2.7 vs 3.5; P = 0.009). There was no significant difference in parotid or splenic SUVmax or lesion to parotid ratios between the two tracers although there was a small difference in lesion to spleen ratios (P = 0.034). Conclusion: There are differences in biodistribution, and in particular liver activity, between 68Ga-PSMA-11 and 99mTc-MIP-1404. Therefore, if 99mTc-MIP-1404 is used to assess eligibility for 177Lu-PSMA-617 therapy, a lower adjusted lesion to liver ratio should be used.

Journal of nuclear medicine : official publication, Society of Nuclear Medicine. 2022 Oct 27 [Epub ahead of print]

Gary J R Cook, Wai-Lup Wong, Bal Sanghera, Stephen Mangar, Amarnath Challapalli, Amit Bahl, Paul Bassett, Darren Leaning, Christian Schmidkonz

King's College London, United Kingdom., Paul Strickland Scanner Centre, United Kingdom., Imperial College Healthcare NHS Trust, United Kingdom., Bristol Cancer Institute, United Kingdom., Statsconsultancy Ltd, United Kingdom., James Cook University Hospital, United Kingdom., University Hospital Erlangen.

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