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An investigational PET imaging agent -- zirconium-89 (⁸⁹Zr)-deferoxamine (DFO)-girentuximab -- identified clear cell renal cell carcinoma (ccRCC) with high sensitivity and specificity, according to results from the phase III ZIRCON trial.

Use of the agent achieved sensitivity and specificity rates of 85.5% (95% CI 79.8-89.8) and 87% (95% CI 78.8-92.3), respectively, reported Peter Mulders, MD, PhD, of the Radboud University Medical Center in Nijmegen, The Netherlands.

Sensitivity and specificity thresholds were exceeded by all three independent readers in the trial presented at the in Milan.

The results suggest that ⁸⁹Zr-DFO-girentuximab improves the identification of primary ccRCC compared to cross-sectional imaging, Mulders told conference attendees, saying it "has the potential to improve management by aiding risk stratification, selection of appropriate patients for treatment, or suggesting where further imaging or biopsy could be indicated."

In explaining the rationale for the study, Mulders pointed out that there is an unmet need for the non-invasive diagnosis and characterization of ccRCC, and that currently "it is difficult to distinguish between benign and malignant renal masses." Furthermore, he noted, 20-30% of resected small renal masses are ultimately determined to be benign.

Thus, he and his colleagues noted, there is a need for accurate, noninvasive methods of pretreatment risk stratification to help guide management.

The novel radiotracer homes in on ccRCC by incorporating girentuximab, a monoclonal antibody that targets carbonic anhydrase IX (CAIX), an enzyme highly expressed in ccRCC. According to Mulders and colleagues, this can aid differentiation between ccRCCs and other renal lesions.

enrolled 332 patients across 36 sites in Europe, North America, and Australia, of whom 300 were included in the safety analysis set (median age 62 years, 71% male) and 284 in the full analysis set. Patients received a single dose of the agent on day 0 and underwent PET/CT imaging on day 5 (± 2 days) prior to surgery. Then within 90 days, all patients had a partial or radical nephrectomy at the investigator's discretion.

Among the evaluable surgical samples, the lesion median size was 3.7 cm, with 62% cT1a (≤4 cm) lesions and 38% cT1b lesions. About 14% of lesions were 2 cm or smaller, and 67% were clear cell lesions.

In order for the study to be declared successful, the 95% CI had to exceed 70% for sensitivity and 68% for specificity by at least two of the three independent readers. These thresholds were exceeded by all three independent readers for the full analysis set, Mulders reported.

In addition to the sensitivity and specificity results for the full analysis set, Mulders and his colleagues also evaluated key secondary endpoints, including positive predictive value, negative predictive value, and accuracy, for which the rates were 93% (95% CI 88-96), 75% (95% CI 66-82), and 86% (95% CI 81.5-89.6), respectively.

For the smaller cT1a tumors, the overall results showed a sensitivity of 85.5% (95% CI 77-91.2) and a specificity of 89.5% (95% CI 78.6-95.2). Sensitivity and specificity thresholds were again exceeded by all three readers.

Mulders and his colleagues reported that treatment-emergent adverse events (TEAEs) were observed in 124 patients, with few considered related or possibly related to ⁸⁹Zr-DFO-girentuximab. Most were considered mild, with 6% having a grade 3 or greater TEAE.

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