A pilot study of PSMA-targeted F-18-DCFPyL PET imaging of patients with adenoid cystic carcinoma

Adenoid cystic carcinoma (ACC) is a rare malignancy of the salivary glands with poor long-term outcomes and with a need for improved imaging and therapeutic options. Prostate-specific membrane antigen (PSMA) expression has been observed in ACC and preliminary studies have demonstrated PET imaging using ⁶⁸Ga-functionalized PSMA agents for positron emission tomography (PET). We aimed to assess the extent of PSMA expression in a collection of archival ACC samples and demonstrate the feasibility of using ¹⁸F-DCFPyL for PSMA PET imaging of ACC. PSMA expression levels were assessed in 77 ACC and 11 unaffected parotid gland control samples by immunohistochemistry and quantified by mean H-score. Three patients with metastatic ACC, who had previously undergone local resection, were imaged with¹⁸F-DCFPyL PSMA PET in a prospective, pilot trial setting. Demographic, oncologic, and treatment history from the PET patient cohort were acquired at the time of imaging. Maximum standardized uptake values (SUV_max) were obtained from representative presumptive metastatic lesions on the ¹⁸F-DCFPyL scans by manual placement of regions-of-interest. PSMA expression was detected in 52% of archival ACC samples, compared to 18% in the unaffected salivary glands. Moderate or high levels of PSMA expression (H-score > 5) were seen in 37% of samples. Radiotracer avid disease was identified on PET imaging of all three patients with tumor SUV_max values of 4.1, 4.0, and 2.0, corresponding to tumor-to-liver ratios of 0.7, 1.0, and 0.4 respectively. We find that PSMA is expressed in a majority of histologic samples from patients with ACC. We also demonstrated the feasibility of ¹⁸F-DCFPyL PSMA-targeted PET imaging in the assessment of ACC. Overall, the tumor uptake on ¹⁸F-DCFPyL PET was modest compared to lesional uptake seen in prostate cancer. Given the potential role of PSMA-targeted agents in the management of ACC, broadening access to PSMA PET through F-18-labeled PSMA PET agents is important. Clinical trials investigating the use of PSMA-targeted radioligand therapies for ACC are underway.