PET Imaging for Monitoring Cellular and Immunotherapy of Cancer.

ABSTRACT: Cancer immunotherapy, including checkpoint blockade and cellular therapy, has become a cornerstone in cancer treatment. However, understanding the factors driving patient response or resistance to these therapies remains challenging. The dynamic interplay between the immune system and tumors requires new approaches for characterization. Biopsies and blood tests provide valuable information, but their limitations have led to increased interest in positron emission tomography (PET)/computed tomography imaging to complement these strategies. The noninvasive nature of PET imaging makes it ideal for monitoring the dynamic tumor immune microenvironment. This review discusses various PET imaging approaches, including immune cell lineage markers, immune functional markers, immune cell metabolism, direct cell labeling, and reporter genes, highlighting their potential in targeted immunotherapies and cell-based approaches. Although PET imaging has limitations, its integration into diagnostic strategies holds promise for improving patient outcomes and accelerating drug development in cancer immunotherapy.

[18F]FMISO-PET imaging reveals the role of hypoxia severity in checkpoint blockade response.

CONTEXT: Hypoxia within the tumor microenvironment is a critical factor influencing the efficacy of immunotherapy, including immune checkpoint inhibition. Insufficient oxygen supply, characteristic of hypoxia, has been recognized as a central determinant in the progression of various cancers. The reemergence of evofosfamide, a hypoxia-activated prodrug, as a potential treatment strategy has sparked interest in addressing the role of hypoxia in immunotherapy response. This investigation sought to understand the kinetics and heterogeneity of tumor hypoxia and their implications in affecting responses to immunotherapeutic interventions with and without evofosfamide. PURPOSE: This study aimed to investigate the influence of hypoxia on immune checkpoint inhibition, evofosfamide monotherapy, and their combination on colorectal cancer (CRC). Employing positron emission tomography (PET) imaging, we developed novel analytical methods to quantify and characterize tumor hypoxia severity and distribution. PROCEDURES: Murine CRC models were longitudinally imaged with [18F]-fluoromisonidazole (FMISO)-PET to quantify tumor hypoxia during checkpoint blockade (anti-CTLA-4 + and anti-PD1 +/- evofosfamide). Metrics including maximum tumor [18F]FMISO uptake (FMISOmax) and mean tumor [18F]FMISO uptake (FMISOmean) were quantified and compared with normal muscle tissue (average muscle FMISO uptake (mAvg) and muscle standard deviation (mSD)). Histogram distributions were used to evaluate heterogeneity of tumor hypoxia. FINDINGS: Severe hypoxia significantly impeded immunotherapy effectiveness consistent with an immunosuppressive microenvironment. Hypoxia-specific PET imaging revealed a striking degree of spatial heterogeneity in tumor hypoxia, with some regions exhibiting significantly more severe hypoxia than others. The study identified FMISOmax as a robust predictor of immunotherapy response, emphasizing the impact of localized severe hypoxia on tumor volume control during therapy. Interestingly, evofosfamide did not directly reduce hypoxia but markedly improved the response to immunotherapy, uncovering an alternative mechanism for its efficacy. CONCLUSIONS: These results enhance our comprehension of the interplay between hypoxia and immune checkpoint inhibition within the tumor microenvironment, offering crucial insights for the development of personalized cancer treatment strategies. Non-invasive hypoxia quantification through molecular imaging evaluating hypoxia severity may be an effective tool in guiding treatment planning, predicting therapy response, and ultimately improving patient outcomes across diverse cancer types and tumor microenvironments. It sets the stage for the translation of these findings into clinical practice, facilitating the optimization of immunotherapy regimens by addressing tumor hypoxia and thereby enhancing the efficacy of cancer treatments.

Evaluation of [89Zr]Zr-DFO-2Rs15d Nanobody for Imaging of HER2-Positive Breast Cancer.

One of the most aggressive forms of breast cancer involves the overexpression of human epidermal growth factor receptor 2 (HER2). HER2 is overexpressed in ∼25% of all breast cancers and is associated with increased proliferation, increased rates of metastasis, and poor prognosis. Treatment for HER2-positive breast cancer has vastly improved since the development of the monoclonal antibody trastuzumab (Herceptin) as well as other biological constructs. However, patients still commonly develop resistance, illustrating the need for newer therapies. Nanobodies have become an important focus for potential development as HER2-targeting imaging agents and therapeutics. Nanobodies have many favorable characteristics, including high stability in heat and nonphysiological pH, while maintaining their low-nanomolar affinity for their designed targets. Specifically, the 2Rs15d nanobody has been developed for targeting HER2 and has been evaluated as a diagnostic imaging agent for single-photon emission computed tomography (SPECT) and positron emission tomography (PET). While a construct of 2Rs15d with the positron emitter 68Ga is currently in phase I clinical trials, the only PET images acquired in preclinical or clinical research have been within 3 h postinjection. We evaluated our in-house produced 2Rs15d nanobody, conjugated with the chelator deferoxamine (DFO), and radiolabeled with 89Zr for PET imaging up to 72 h postinjection. [89Zr]Zr-DFO-2Rs15d demonstrated high stability in both phosphate-buffered saline (PBS) and human serum. Cell binding studies showed high binding and specificity for HER2, as well as prominent internalization. Our in vivo PET imaging confirmed high-quality visualization of HER2-positive tumors up to 72 h postinjection, whereas HER2-negative tumors were not visualized. Subsequent biodistribution studies quantitatively supported the significant HER2-positive tumor uptake compared to the negative control. Our studies fill an important gap in understanding the imaging and binding properties of the 2Rs15d nanobody at extended time points. As many therapeutic radioisotopes have single or multiday half-lives, this information will directly benefit the potential of the radiotherapy development of 2Rs15d for HER2-positive breast cancer patients.

Exposure of progressive immune dysfunction by SARS-CoV-2 mRNA vaccination in patients with chronic lymphocytic leukemia: A prospective cohort study.

BACKGROUND: Patients with chronic lymphocytic leukemia (CLL) have reduced seroconversion rates and lower binding antibody (Ab) and neutralizing antibody (NAb) titers than healthy individuals following Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) mRNA vaccination. Here, we dissected vaccine-mediated humoral and cellular responses to understand the mechanisms underlying CLL-induced immune dysfunction. METHODS AND FINDINGS: We performed a prospective observational study in SARS-CoV-2 infection-naïve CLL patients (n = 95) and healthy controls (n = 30) who were vaccinated between December 2020 and June 2021. Sixty-one CLL patients and 27 healthy controls received 2 doses of the Pfizer-BioNTech BNT162b2 vaccine, while 34 CLL patients and 3 healthy controls received 2 doses of the Moderna mRNA-1273 vaccine. The median time to analysis was 38 days (IQR, 27 to 83) for CLL patients and 36 days (IQR, 28 to 57) for healthy controls. Testing plasma samples for SARS-CoV-2 anti-spike and receptor-binding domain Abs by enzyme-linked immunosorbent assay (ELISA), we found that all healthy controls seroconverted to both antigens, while CLL patients had lower response rates (68% and 54%) as well as lower median titers (23-fold and 30-fold; both p < 0.001). Similarly, NAb responses against the then prevalent D614G and Delta SARS-CoV-2 variants were detected in 97% and 93% of controls, respectively, but in only 42% and 38% of CLL patients, who also exhibited >23-fold and >17-fold lower median NAb titers (both p < 0.001). Interestingly, 26% of CLL patients failed to develop NAbs but had high-titer binding Abs that preferentially reacted with the S2 subunit of the SARS-CoV-2 spike. Since these patients were also seropositive for endemic human coronaviruses (HCoVs), these responses likely reflect cross-reactive HCoV Abs rather than vaccine-induced de novo responses. CLL disease status, advanced Rai stage (III-IV), elevated serum beta-2 microglobulin levels (ß2m >2.4 mg/L), prior therapy, anti-CD20 immunotherapy (<12 months), and intravenous immunoglobulin (IVIg) prophylaxis were all predictive of an inability to mount SARS-CoV-2 NAbs (all p ≤ 0.03). T cell response rates determined for a subset of participants were 2.8-fold lower for CLL patients compared to healthy controls (0.05, 95% CI 0.01 to 0.27, p < 0.001), with reduced intracellular IFNγ staining (p = 0.03) and effector polyfunctionality (p < 0.001) observed in CD4+ but not in CD8+ T cells. Surprisingly, in treatment-naïve CLL patients, BNT162b2 vaccination was identified as an independent negative risk factor for NAb generation (5.8, 95% CI 1.6 to 27, p = 0.006). CLL patients who received mRNA-1273 had 12-fold higher (p < 0.001) NAb titers and 1.7-fold higher (6.5, 95% CI 1.3 to 32, p = 0.02) response rates than BNT162b2 vaccinees despite similar disease characteristics. The absence of detectable NAbs in CLL patients was associated with reduced naïve CD4+ T cells (p = 0.03) and increased CD8+ effector memory T cells (p = 0.006). Limitations of the study were that not all participants were subjected to the same immune analyses and that pre-vaccination samples were not available. CONCLUSIONS: CLL pathogenesis is characterized by a progressive loss of adaptive immune functions, including in most treatment-naïve patients, with preexisting memory being preserved longer than the capacity to mount responses to new antigens. In addition, higher NAb titers and response rates identify mRNA-1273 as a superior vaccine for CLL patients.

Imaging of Activated T Cells.

The adaptive immune response plays a critical role in detecting, eliminating, and creating a memory toward foreign pathogens and malignant cells. Demonstration of the specific and effective target killing of T cells in cancer has reignited interest in the study and therapeutic manipulation of the interaction between tumor and immune system. To both improve therapeutic efficacy and reduce adverse events, accurate monitoring of the activation of T cells is required. Several approaches to monitoring not just the presence, but importantly the activation, of T cells have been developed. Here, we review the recent advances in T-cell activation imaging and future directions for potential implementation into clinical utility.

Preclinical PET Imaging of Granzyme B Shows Promotion of Immunological Response Following Combination Paclitaxel and Immune Checkpoint Inhibition in Triple Negative Breast Cancer.

Advancements in monitoring and predicting of patient-specific response of triple negative breast cancer (TNBC) to immunotherapy (IMT) with and without chemotherapy are needed. Using granzyme B-specific positron emission tomography (GZP-PET) imaging, we aimed to monitor changes in effector cell activation in response to IMT with chemotherapy in TNBC. TNBC mouse models received the paclitaxel (PTX) ± immune checkpoint inhibitors anti-programmed death 1 (anti-PD1) and anti-cytotoxic T-lymphocyte 4 (anti-CTLA4). GZP-PET imaging was performed on treatment days 0, 3, and 6. Mean standard uptake value (SUVmean), effector cell fractions, and SUV histograms were compared. Mice were sacrificed at early imaging timepoints for cytokine and histological analyses. GZP-PET imaging data revealed differences prior to tumor volume changes. By day six, responders had SUVmean ≥ 2.2-fold higher (p < 0.0037) and effector cell fractions ≥ 1.9-fold higher (p = 0.03) compared to non-responders. IMT/PTX resulted in a significantly different SUV distribution compared to control, indicating broader distribution of activated intratumoral T-cells. IMT/PTX resulted in significantly more necrotic tumor tissue and increased levels of IL-2, 4, and 12 compared to control. Results implicate immunogenic cell death through upregulation of key Th1/Th2 cytokines by IMT/PTX. Noninvasive PET imaging can provide data on the TNBC tumor microenvironment, specifically intratumoral effector cell activation, predicting response to IMT plus chemotherapy.

Comprehensive mapping of SARS-CoV-2 peptide epitopes for development of a highly sensitive serological test for total and neutralizing antibodies.

Quantification of the anti-SARS-CoV-2 antibody response has proven to be a prominent diagnostic tool during the COVID-19 pandemic. Antibody measurements have aided in the determination of humoral protection following infection or vaccination and will likely be essential for predicting the prevalence of population level immunity over the next several years. Despite widespread use, current tests remain limited in part, because antibody capture is accomplished through the use of complete spike and nucleocapsid proteins that contain significant regions of overlap with common circulating coronaviruses. To address this limitation, a unique epitope display platform utilizing monovalent display and protease-driven capture of peptide epitopes was used to select high affinity peptides. A single round of selection using this strategy with COVID-19 positive patient plasma samples revealed surprising differences and specific patterns in the antigenicity of SARS-CoV-2 proteins, especially the spike protein. Putative epitopes were assayed for specificity with convalescent and control samples, and the individual binding kinetics of peptides were also determined. A subset of prioritized peptides was used to develop an antibody diagnostic assay that showed low cross reactivity while detecting 37% more positive antibody cases than a gold standard FDA EUA test. Finally, a subset of peptides were compared with serum neutralization activity to establish a 2 peptide assay that strongly correlates with neutralization. Together, these data demonstrate a novel phage display method that is capable of comprehensively and rapidly mapping patient viral antibody responses and selecting high affinity public epitopes for the diagnosis of humoral immunity.

SARS-CoV-2 mRNA vaccination exposes progressive adaptive immune dysfunction in patients with chronic lymphocytic leukemia.

Chronic lymphocytic leukemia (CLL) patients have lower seroconversion rates and antibody titers following SARS-CoV-2 vaccination, but the reasons for this diminished response are poorly understood. Here, we studied humoral and cellular responses in 95 CLL patients and 30 healthy controls after two BNT162b2 or mRNA-2173 mRNA immunizations. We found that 42% of CLL vaccinees developed SARS-CoV-2-specific binding and neutralizing antibodies (NAbs), while 32% had no response. Interestingly, 26% were seropositive, but had no detectable NAbs, suggesting the maintenance of pre-existing endemic human coronavirus-specific antibodies that cross-react with the S2 domain of the SARS-CoV-2 spike. These individuals had more advanced disease. In treatment-naïve CLL patients, mRNA-2173 induced 12-fold higher NAb titers and 1.7-fold higher response rates than BNT162b2. These data reveal a graded loss of immune function, with pre-existing memory being preserved longer than the capacity to respond to new antigens, and identify mRNA-2173 as a superior vaccine for CLL patients.

18F-FMISO PET Imaging Identifies Hypoxia and Immunosuppressive Tumor Microenvironments and Guides Targeted Evofosfamide Therapy in Tumors Refractory to PD-1 and CTLA-4 Inhibition.

PURPOSE: Hypoxia is a common characteristic of many tumor microenvironments, and it has been shown to promote suppression of antitumor immunity. Despite strong biological rationale, longitudinal correlation of hypoxia and response to immunotherapy has not been investigated. EXPERIMENTAL DESIGN: In this study, we probed the tumor and its surrounding microenvironment with 18F-FMISO PET imaging to noninvasively quantify tumor hypoxia in vivo prior to and during PD-1 and CTLA-4 checkpoint blockade in preclinical models of breast and colon cancer. RESULTS: Longitudinal imaging identified hypoxia as an early predictive biomarker of therapeutic response (prior to anatomic changes in tumor volume) with a decreasing standard uptake value (SUV) ratio in tumors that effectively respond to therapy. PET signal correlated with ex vivo markers of tumor immune response including cytokines (IFNγ, GZMB, and TNF), damage-associated molecular pattern receptors (TLR2/4), and immune cell populations (macrophages, dendritic cells, and cytotoxic T cells). Responding tumors were marked by increased inflammation that were spatially distinct from hypoxic regions, providing a mechanistic understanding of the immune signaling pathways activated. To exploit image-guided combination therapy, hypoxia signal from PET imaging was used to guide the addition of a hypoxia targeted treatment to nonresponsive tumors, which ultimately provided therapeutic synergy and rescued response as determined by longitudinal changes in tumor volume. CONCLUSIONS: The results generated from this work provide an immediately translatable paradigm for measuring and targeting hypoxia to increase response to immune checkpoint therapy and using hypoxia imaging to guide combinatory therapies.

T cells armed with C-X-C chemokine receptor type 6 enhance adoptive cell therapy for pancreatic tumours.

The efficacy of adoptive cell therapy for solid tumours is hampered by the poor accumulation of the transferred T cells in tumour tissue. Here, we show that forced expression of C-X-C chemokine receptor type 6 (whose ligand is highly expressed by human and murine pancreatic cancer cells and tumour-infiltrating immune cells) in antigen-specific T cells enhanced the recognition and lysis of pancreatic cancer cells and the efficacy of adoptive cell therapy for pancreatic cancer. In mice with subcutaneous pancreatic tumours treated with T cells with either a transgenic T-cell receptor or a murine chimeric antigen receptor targeting the tumour-associated antigen epithelial cell adhesion molecule, and in mice with orthotopic pancreatic tumours or patient-derived xenografts treated with T cells expressing a chimeric antigen receptor targeting mesothelin, the T cells exhibited enhanced intratumoral accumulation, exerted sustained anti-tumoral activity and prolonged animal survival only when co-expressing C-X-C chemokine receptor type 6. Arming tumour-specific T cells with tumour-specific chemokine receptors may represent a promising strategy for the realization of adoptive cell therapy for solid tumours.

[89Zr]-Pertuzumab PET Imaging Reveals Paclitaxel Treatment Efficacy Is Positively Correlated with HER2 Expression in Human Breast Cancer Xenograft Mouse Models.

Paclitaxel (PTX) treatment efficacy varies in breast cancer, yet the underlying mechanism for variable response remains unclear. This study evaluates whether human epidermal growth factor receptor 2 (HER2) expression level utilizing advanced molecular positron emission tomography (PET) imaging is correlated with PTX treatment efficacy in preclinical mouse models of HER2+ breast cancer. HER2 positive (BT474, MDA-MB-361), or HER2 negative (MDA-MB-231) breast cancer cells were subcutaneously injected into athymic nude mice and PTX (15 mg/kg) was administrated. In vivo HER2 expression was quantified through [89Zr]-pertuzumab PET/CT imaging. PTX treatment response was quantified by [18F]-fluorodeoxyglucose ([18F]-FDG) PET/CT imaging. Spearman's correlation, Kendall's tau, Kolmogorov-Smirnov test, and ANOVA were used for statistical analysis. [89Zr]-pertuzumab mean standard uptake values (SUVmean) of BT474 tumors were 4.9 ± 1.5, MDA-MB-361 tumors were 1.4 ± 0.2, and MDA-MB-231 (HER2-) tumors were 1.1 ± 0.4. [18F]-FDG SUVmean changes were negatively correlated with [89Zr]-pertuzumab SUVmean (r = -0.5887, p = 0.0030). The baseline [18F]-FDG SUVmean was negatively correlated with initial [89Zr]-pertuzumab SUVmean (r = -0.6852, p = 0.0002). This study shows PTX treatment efficacy is positively correlated with HER2 expression level in human breast cancer mouse models. Molecular imaging provides a non-invasive approach to quantify biological interactions, which will help in identifying chemotherapy responders and potentially enhance clinical decision-making.

Imaging for Response Assessment in Cancer Clinical Trials.

The use of biomarkers is integral to the routine management of cancer patients, including diagnosis of disease, clinical staging and response to therapeutic intervention. Advanced imaging metrics with computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) are used to assess response during new drug development and in cancer research for predictive metrics of response. Key components and challenges to identifying an appropriate imaging biomarker are selection of integral vs integrated biomarkers, choosing an appropriate endpoint and modality, and standardization of the imaging biomarkers for cooperative and multicenter trials. Imaging biomarkers lean on the original proposed quantified metrics derived from imaging such as tumor size or longest dimension, with the most commonly implemented metrics in clinical trials coming from the Response Evaluation Criteria in Solid Tumors (RECIST) criteria, and then adapted versions such as immune-RECIST (iRECIST) and Positron Emission Tomography Response Criteria in Solid Tumors (PERCIST) for immunotherapy response and PET imaging, respectively. There have been many widely adopted biomarkers in clinical trials derived from MRI including metrics that describe cellularity and vascularity from diffusion-weighted (DW)-MRI apparent diffusion coefficient (ADC) and Dynamic Susceptibility Contrast (DSC) or dynamic contrast enhanced (DCE)-MRI (Ktrans, relative cerebral blood volume (rCBV)), respectively. Furthermore, Fluorodexoyglucose (FDG), fluorothymidine (FLT), and fluoromisonidazole (FMISO)-PET imaging, which describe molecular markers of glucose metabolism, proliferation and hypoxia have been implemented into various cancer types to assess therapeutic response to a wide variety of targeted- and chemotherapies. Recently, there have been many functional and molecular novel imaging biomarkers that are being developed that are rapidly being integrated into clinical trials (with anticipation of being implemented into clinical workflow in the future), such as artificial intelligence (AI) and machine learning computational strategies, antibody and peptide specific molecular imaging, and advanced diffusion MRI. These include prostate-specific membrane antigen (PSMA) and trastuzumab-PET, vascular tumor burden extracted from contrast-enhanced CT, diffusion kurtosis imaging, and CD8 or Granzyme B PET imaging. Further excitement surrounds theranostic procedures such as the combination of 68Ga/111In- and 177Lu-DOTATATE to use integral biomarkers to direct care and personalize therapy. However, there are many challenges in the implementation of imaging biomarkers that remains, including understand the accuracy, repeatability and reproducibility of both acquisition and analysis of these imaging biomarkers. Despite the challenges associated with the biological and technical validation of novel imaging biomarkers, a distinct roadmap has been created that is being implemented into many clinical trials to advance the development and implementation to create specific and sensitive novel imaging biomarkers of therapeutic response to continue to transform medical oncology.

The Effectiveness of Checkpoint Inhibitor Combinations and Administration Timing Can Be Measured by Granzyme B PET Imaging.

PURPOSE: The lack of a timely and reliable measure of response to cancer immunotherapy has confounded understanding of mechanisms of resistance and subsequent therapeutic advancement. We hypothesized that PET imaging of granzyme B using a targeted peptide, GZP, could be utilized for early response assessment across many checkpoint inhibitor combinations, and that GZP uptake could be compared between therapeutic regimens and dosing schedules as an early biomarker of relative efficacy. EXPERIMENTAL DESIGN: Two models, MC38 and CT26, were treated with a series of checkpoint inhibitors. GZP PET imaging was performed to assess tumoral GZP uptake, and tumor volume changes were subsequently monitored to determine response. The average GZP PET uptake and response of each treatment group were correlated to evaluate the utility of GZP PET for comparing therapeutic efficacy. RESULTS: In both tumor models, GZP PET imaging was highly accurate for predicting response, with 93% sensitivity and 94% negative predictive value. Mean tumoral GZP signal intensity of treatment groups linearly correlated with percent response across all therapies and schedules. Moreover, GZP PET correctly predicted that sequential dose scheduling of PD-1 and CTLA-4 targeted therapies demonstrates comparative efficacy to concurrent administration. CONCLUSIONS: Granzyme B quantification is a highly sensitive and specific early measure of therapeutic efficacy for checkpoint inhibitor regimens. This work provides evidence that GZP PET imaging may be useful for rapid assessment of therapeutic efficacy in the context of clinical trials for both novel drugs as well as dosing regimens.

Specific 18F-FDHT Accumulation in Human Prostate Cancer Xenograft Murine Models Is Facilitated by Prebinding to Sex Hormone-Binding Globulin.

Tremendous efforts are currently dedicated to the development of novel therapies targeting the androgen receptor (AR), the major driver of prostate cancer disease and its progression to castration resistance. The ability to noninvasively interrogate AR expression over time in murine models of prostate cancer would permit longitudinal preclinical analysis of novel compounds that could not otherwise be accomplished ex vivo. Although PET imaging with 16ß-18F-fluoro-5α-dihydrotestosterone (18F-FDHT) has successfully quantified AR levels clinically, no rodent model of 18F-FDHT imaging has been reported so far. One difference between humans and rodents is the absence in the latter of the sex hormone-binding globulin (SHBG), a glycoprotein that binds to testosterone in the bloodstream, Here, we explore the role of SHBG in developing a working model of rodent AR imaging. Methods: Three human prostate cancer cell lines and xenografts (LNCaP, 22Rv1, and PC3) were used to examine the uptake of free 18F-FDHT and SHBG-bound 18F-FDHT. Both ligands were examined for stability and competitive binding to AR over time in vitro before in vivo studies. PET/CT imaging was used to dynamically measure the uptake of both tracers over 4 h, whereas specificity was determined by competitive binding with the AR antagonist enzalutamide. Results: AR levels correlated with the uptake of both 18F-FDHT and SHBG-18F-FDHT in prostate cancer cell lines. Interestingly, whereas both free and SHBG-bound 18F-FDHT had a similar cellular accumulation at 1 and 2.5 h, SHBG-18F-FDHT accumulated at significantly higher levels after 4 h-evidence that receptor-mediated uptake of SHBG accounted for later time-point differences. This observation was also seen in 22Rv1 tumor-bearing mice, in which SHBG-18F-FDHT exhibited a significantly increased uptake (average tumor-to-background ratio [TBR], 1.62 ± 0.62) in comparison to unbound 18F-FDHT (TBR, 0.81 ± 0.08) at 4 h. Furthermore, the specificity of the SHBG-18F-FDHT accumulation at 4 h was demonstrated by a reduced tumor uptake after AR blockade with enzalutamide (TBR, 1.07 ± 0.13). Conclusion: Prebinding of 18F-FDHT to SHBG allows accurate and quantitative PET imaging of AR levels in murine models of prostate cancer. This procedure may permit the use of PET imaging to study the longitudinal effects of AR-targeting therapies, accelerating novel-drug development.

Phage Display Selection, In Vitro Characterization, and Correlative PET Imaging of a Novel HER3 Peptide.

PURPOSE: HER3 (ERBB3) is a receptor tyrosine kinase that is implicated in treatment resistance across multiple cancers, including those of the breast, lung, and prostate. Overexpression of HER3 following targeted therapy can occur rapidly and heterogeneously both within a single lesion and across sites of metastasis, making protein quantification by biopsy highly challenging. A global, non-invasive methodology such as positron emission tomography (PET) imaging can permit serial quantification of HER3, providing a useful approach to monitor HER3 expression across the entire tumor burden both prior to and following treatment. PET imaging of HER3 expression may permit a more personalized approach to targeted therapy by allowing for detection of HER3-mediated resistance, in addition to informing clinical trial patient selection for novel therapies targeting HER3. PROCEDURES: Phage display selection targeting the HER3 extracellular domain was performed in order to develop a peptide with optimal blood clearance and highly accurate HER3 quantification. RESULTS: The selection converged to a consensus peptide sequence that was subsequently found to bind HER3 with an affinity of 270 ± 151 nM. The peptide, termed HER3P1, was bound with high selectivity to HER3 over other similar receptor tyrosine kinases such as EGFR and HER2. Furthermore, HER3P1 was able to distinguish between high and low HER3-expressing cells in vitro. The peptide was radiolabeled with Ga-68 and demonstrated to specifically bind HER3 by in vivo PET imaging. Uptake of [68Ga]HER3P1 was highly specific for HER3-positive tumors, with tumor-to-background ratios ranging from 1.59-3.32, compared to those of HER3-negative tumors, ranging from 0.84-0.93. The uptake of [68Ga]HER3P1 also demonstrated high (P < 0.001) correlation with protein expression as quantified by Western blot and confirmed by biodistribution. CONCLUSIONS: HER3P1 accurately quantifies expression of HER3 by PET imaging and has potential utility as a clinical imaging agent.

Granzyme B PET Imaging as a Predictive Biomarker of Immunotherapy Response.

While cancer immunotherapy can produce dramatic responses, only a minority of patients respond to treatment. Reliable response biomarkers are needed to identify responders, and conventional imaging modalities have not proved adequate. Here, we provide a preclinical proof of concept for the use of granzyme B, a downstream effector of tumoral cytotoxic T cells, as an early biomarker for tumors responding to immunotherapy. We designed novel PET imaging probes for the murine and human granzyme B isoforms that specifically and quantitatively bind granzyme B. Immunotherapy-treated mice were imaged prior to therapy-induced tumor volume reduction. Imaging distinguished treated responders from nonresponders with excellent predictive ability. To assess the clinical value of a granzyme B imaging paradigm, biopsy specimens from melanoma patients on checkpoint inhibitor therapy were analyzed. A marked differential in granzyme B expression was observed between treated responders and nonresponders. Additionally, our human probe was able to specifically detect granzyme B expression in human samples, providing a clear candidate for clinical application. Overall, our results suggest granzyme B PET imaging can serve as a quantitatively useful predictive biomarker for efficacious responses to cancer immunotherapy. Cancer Res; 77(9); 2318-27. ©2017 AACR.

Quantitative CD3 PET Imaging Predicts Tumor Growth Response to Anti-CTLA-4 Therapy.

Immune checkpoint inhibitors have made rapid advances, resulting in multiple Food and Drug Administration-approved therapeutics that have markedly improved survival. However, these benefits are limited to a minority subpopulation that achieves a response. Predicting which patients are most likely to benefit would be valuable for individual therapy optimization. T-cell markers such as CD3-by examining active recruitment of the T cells responsible for cancer-cell death-represent a more direct approach to monitoring tumor immune response than pretreatment biopsy or genetic screening. This approach could be especially effective as numerous different therapeutic strategies emerge, decreasing the need for drug-specific biomarkers and instead focusing on T-cell infiltration, which has been previously correlated with treatment response. METHODS: A CD3 PET imaging agent targeting T cells was synthesized to test the role of such imaging as a predictive marker. The 89Zr-p-isothiocyanatobenzyl-deferoxamine-CD3 PET probe was assessed in a murine tumor xenograft model of anti-cytotoxic T-lymphocyte antigen-4 (CTLA-4) immunotherapy of colon cancer. RESULTS: Imaging on day 14 revealed 2 distinct groups of mice stratified by PET signal intensity. Although there was no significant difference in tumor volume on the day of imaging, in the high-uptake group subsequent measurements revealed significantly smaller tumors than in either the low-uptake group or the untreated controls. In contrast, there was no significant difference in the size of tumors between the low-uptake and untreated control mice. CONCLUSION: These findings indicate that high CD3 PET uptake in the anti-CTLA-4-treated mice correlated with subsequent reduced tumor volume and was a predictive biomarker of response.

Multiple Bacteriophage Selection Strategies for Improved Affinity of a Peptide Targeting ERBB2.

Due to the heterogeneity of ERBB2-expression between tumors and over the course of treatment, a non-invasive molecular imaging agent is needed to accurately detect overall ERBB2 status. Peptides are a highly advantageous platform for molecular imaging, since they have excellent tumor penetration and rapid pharmacokinetics. One limitation of peptides however, is their traditionally low target affinity, and consequently, tumor uptake. The peptide KCCYSL was previously selected from a bacteriophage (phage) display library to bind ERBB2 and did so with moderate affinity of 295 nM. In order to enhance tumor uptake and clinical utility of the peptide, a novel phage microlibrary was created by flanking the parent sequence with random amino acids, followed by reselection using parallel strategies for high affinity and specific ERBB2 binding in an attempt to affinity maturate the peptide. One limitation of traditional phage display selections is difficulty in releasing the highest affinity phages from the target by incubation of acidic buffer. In an attempt to recover high affinity second-generation peptides from the ERBB2 microlibrary, two elution strategies, sonication and target elution, were undertaken. Sonication resulted in an approximately 50-fold enhancement in recovered phage per round of selection in comparison to target elution. Despite the differences in elution efficiency, the affinities of phage-displayed peptides selected from either strategy were relatively similar. Although both selections yielded peptides with significantly improved affinity in comparison to KCCYSL, the improvements were modest, most likely because the parental peptide binding cannot be improved by additional amino acids.

Development of a peptide by phage display for SPECT imaging of resistance-susceptible breast cancer.

Personalized medicine is at the forefront of cancer diagnosis and therapy. Molecularly targeted therapies such as trastuzumab and tamoxifen have enhanced prognosis of patients with cancers expressing ERBB2 and the estrogen receptor, respectively. One obstacle to targeted therapy is the development of resistance. A targeted peptide that could distinguish resistance-susceptible cancer would aid in treatment. BT-474 human breast cancer cells can be resistant to both tamoxifen and trastuzumab, and may serve as a model for malignancies in which targeted therapy may not work. Bacteriophage (phage) display is a combinatorial technology that has been used to isolate peptides that target a specific cancer subtype. It was hypothesized that in vivo phage display could be used to select a peptide for SPECT imaging of BT-474 human breast cancer xenografts. A phage library displaying random 15 amino acid peptides was subjected to four rounds of selection, after which 14 clones were analyzed for BT-474 binding and specificity. One phage clone, 51, demonstrated superior binding and specificity, and the displayed peptide was synthesized for in vitro characterization. Peptide 51 bound specifically to BT-474 cells with an EC50 = 2.33 µM and was synthesized as a DOTA-conjugated peptide and radiolabeled with (111)In for in vitro and in vivo analysis. The radiolabeled peptide exhibited an IC50 = 16.1 nM to BT-474 cells and its biodistribution and SPECT imaging in BT-474 xenografted mice was analyzed. Although tumor uptake was moderate at 0.11% ID/g, SPECT imaging revealed a distinct tumor vasculature binding pattern. It was discovered that peptide 51 had an identical 5 amino acid N-terminal sequence to a peptide, V1, which bound to Nrp1, a tumor vasculature protein. Peptide 51 and V1 were examined for binding to target cells, and 51 bound both target and endothelial cells, while V1 only bound endothelial cells. Truncated versions of 51 did not bind BT-474 cells, demonstrating that the targeting ability of 51 was independent of the homologous V1 sequence. These results demonstrate that in vivo phage display can effectively identify a peptide that specifically targets a breast cancer cell line that is susceptible to targeted therapy resistance.