Imaging alpha(v)beta(3) integrin expression in skeletal metastases with Tc-99m-maraciclatide single-photon emission computed tomography: detection and therapy response assessment

Purpose Osteoclast activity is an important factor in the pathogenesis of skeletal metastases and is a potential therapeutic target. This study aimed to determine if selective uptake of Tc-99m-maraciclatide, a radiopharmaceutical targeting alpha(v)beta(3) integrin, occurs in prostate cancer (PCa) bone metastases and to observe the changes following systemic therapy. Methods The study group comprised 17 men with bone-predominant metastatic PCa who underwent whole-body planar and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging with Tc-99m-maraciclatide before (n = 17) and 12 weeks after (n = 11) starting treatment with abiraterone. Tumour to normal bone (T:N) ratios, tumour to muscle (T:M) ratios and CT Hounsfield units (HU) were measured in up to five target metastases in each subject. An oncologist blinded to study scans assessed clinical responses up to 24 weeks using conventional criteria. Results Before treatment, metastases showed specific Tc-99m-maraciclatide accumulation (mean planar T:N and T:M ratios 1.43 and 3.06; SPECT T:N and T:M ratios 3.1 and 5.19, respectively). Baseline sclerotic lesions (389-740 HU) showed lower T:M ratios (4.22 vs. 7.04, p = 0.02) than less sclerotic/lytic lesions (46-381 HU). Patients with progressive disease (PD; n = 5) showed increased planar T:N and T:M ratios (0.29 and 12.1%, respectively) and SPECT T:N and T:M ratios (11.9 and 20.2%, respectively). Patients without progression showed decreased planar T:N and T:M ratios (0.27 and -8.0%, p = 1.0 and 0.044, respectively) and SPECT T:N and T:M ratios (-21.9, and -27.2%, p = 0.3 and 0.036, respectively). The percentage change in CT HU was inversely correlated with the percentage change in SPECT T:M ratios (r = -0.59, p = 0.006). Conclusions Tc-99m-maraciclatide accumulates in PCa bone metastases in keeping with increased alpha(v)beta(3) integrin expression. Greater activity in metastases with lower CT density suggests that uptake is related to osteoclast activity. Changes in planar and SPECT T:M ratios after 12 weeks of treatment differed between patients with and without PD and Tc-99m-maraciclatide imaging may be a potential method for assessing early response.