High sensitivity imaging tools could provide a more holistic view of target antigen expression to improve the identification of patients who might benefit from cancer immunotherapy. We developed for immunoPET a novel recombinant human IgG1 (termed C4) that potently binds an extracellular epitope on human and mouse PD-L1 and radiolabeled the antibody with zirconium-89. Small animal PET/CT studies showed that 89Zr-C4 detected antigen levels on a patient derived xenograft (PDX) established from a non-small-cell lung cancer (NSCLC) patient before an 8-month response to anti-PD-1 and anti-CTLA4 therapy. Importantly, the concentration of antigen is beneath the detection limit of previously developed anti-PD-L1 radiotracers, including radiolabeled atezolizumab. We also show that 89Zr-C4 can specifically detect antigen in human NSCLC and prostate cancer models endogenously expressing a broad range of PD-L1. 89Zr-C4 detects mouse PD-L1 expression changes in immunocompetent mice, suggesting that endogenous PD-1/2 will not confound human imaging. Lastly, we found that 89Zr-C4 could detect acute changes in tumor expression of PD-L1 due to standard of care chemotherapies. In summary, we present evidence that low levels of PD-L1 in clinically relevant cancer models can be imaged with immunoPET using a novel recombinant human antibody.

No abstract

Purpose: The recent emergence of radioligand therapies for cancer treatment has increased enthusiasm for developing new theranostic strategies coupling both imaging and cytotoxicity in the same entity. In this study, we evaluated whether CUB domain containing protein 1 (CDCP1), a single-pass transmembrane protein highly overexpressed in diverse human cancers, might be a target for cancer theranostics.Experimental Design: The ectodomain of CDCP1 was targeted using radiolabeled forms of 4A06, a potent and specific recombinant human antibody that we developed. Imaging and antitumor assessment studies were performed in animal models of pancreatic cancer, including two patient-derived xenograft models we developed for this study. For antitumor assessment studies, the endpoints were death due to tumor volume >3,000 mm 3 or ≥20% loss in body weight. Specific tracer binding or antitumor effects were assessed with an unpaired, two-tailed Student t test and survival advantages were assessed with a log rank (Mantel-Cox) test. Differences at the 95% confidence level were interpreted to be significant.Results: 89 Zr-4A06 detected a broad dynamic range of full length or cleaved CDCP1 expression on seven human pancreatic cancer tumors (n ¼ 4/tumor). Treating mice with single or fractionated doses of 177 Lu-4A06 significantly reduced pancreatic cancer tumor volume compared with mice receiving vehicle or unlabeled 4A06 (n ¼ 8; P < 0.01). A single dose of 225 Ac-4A06 also inhibited tumor growth, although the effect was less profound compared with 177 Lu-4A06 (n ¼ 8; P < 0.01). A significant survival advantage was imparted by 225 Ac-4A06 (HR ¼ 2.56; P < 0.05).Conclusions: These data establish that CDCP1 can be exploited for theranostics, a finding with widespread implications given its breadth of overexpression in cancer.

The swell of experimental imaging technologies to noninvasively measure immune checkpoint protein expression presents the opportunity for rigorous comparative studies toward identifying a gold standard. Zr-atezolizumab is currently in man, and early data show tumor targeting but also abundant uptake in several normal tissues. Therefore, we conducted a reverse translational study both to understand if tumor to normal tissue ratios forZr-atezolizumab could be improved and to make direct comparisons to Zr-C4, a radiotracer that we showed can detect a large dynamic range of tumor-associated PD-L1 expression. PET/CT and biodistribution studies in tumor bearing immunocompetent and nu/nu mice revealed that high specific activityZr-atezolizumab (∼2 μCi/μg) binds to PD-L1 on tumors but also results in very high uptake in many normal mouse tissues, as expected. Unexpectedly, Zr-atezolizumab uptake was generally higher in normal mouse tissues compared toZr-C4 and lower in H1975, a tumor model with modest PD-L1 expression. Also unexpectedly, reducing the specific activity at least 15-fold suppressed Zr-atezo uptake in normal mouse tissues but increased tumor uptake to levels observed with high specific activityZr-C4. In summary, these data reveal that low specific activity Zr-atezo may be necessary for accurately measuring PD-L1 in the tumor microenvironment, assuming a threshold can be identified that preferentially suppresses binding in normal tissues without reducing binding to tumors with abundant expression. Alternatively, high specific activity approaches likeZr-C4 PET may be simpler to implement clinically to measure the broad dynamic range of PD-L1 expression known to manifest among tumors.