FAPI PET Signal in Hibernoma Reflects FAP Expression in Tumor Vasculature Cells

ABSTRACT

A 43-year-old man with a growing mass in the right groin, concerned for liposarcoma, underwent MRI and 68 Ga-fibroblast activation protein inhibitor (FAPI)-46 PET/CT before surgery. Fibroblast activation protein inhibitor PET/CT demonstrated increased uptake (SUV max , 3.2) predominantly in the solid portion, where MRI showed gadolinium enhancement. The patient subsequently underwent surgery and was diagnosed with hibernoma. The immunohistochemistry of the tumor revealed the fibroblast activation protein expression in the fibrovascular network and myofibroblastic cells of the tumor. This case suggests that the FAPI uptake can be affected by the vascular cells, and thus, a careful interpretation of the FAPI PET signal may be needed.

Fibroblast Activation Protein Expression in Sarcomas

Objectives

Fibroblast activation protein alpha (FAP) is highly expressed by cancer-associated fibroblasts in multiple epithelial cancers. The aim of this study was to characterize FAP expression in sarcomas to explore its potential utility as a diagnostic and therapeutic target and prognostic biomarker in sarcomas.

Methods

Available tissue samples from patients with bone or soft tissue tumors were identified at the University of California, Los Angeles. FAP expression was evaluated via immunohistochemistry (IHC) in tumor samples (n = 63), adjacent normal tissues (n = 30), and positive controls (n = 2) using semiquantitative systems for intensity (0 = negative; 1 = weak; 2 = moderate; and 3 = strong) and density (none, <25%, 25-75%; >75%) in stromal and tumor/nonstromal cells and using a qualitative overall score (not detected, low, medium, and high). Additionally, RNA sequencing data in publicly available databases were utilized to compare FAP expression in samples (n = 10,626) from various cancer types and evaluate the association between FAP expression and overall survival (OS) in sarcoma (n = 168).

Results

The majority of tumor samples had FAP IHC intensity scores ≥2 and density scores ≥25% for stromal cells (77.7%) and tumor cells (50.7%). All desmoid fibromatosis, myxofibrosarcoma, solitary fibrous tumor, and undifferentiated pleomorphic sarcoma samples had medium or high FAP overall scores. Sarcomas were among cancer types with the highest mean FAP expression by RNA sequencing. There was no significant difference in OS in patients with sarcoma with low versus high FAP expression.

Conclusion

The majority of the sarcoma samples showed FAP expression by both stromal and tumor/nonstromal cells. Further investigation of FAP as a potential diagnostic and therapeutic target in sarcomas is warranted.

Fibroblast Activation Protein Inhibitor Theranostics

Fibroblast activation protein (FAP)-radioligand therapy might be effective in some patients without being curative. FAP-radioligands deliver ionizing radiation directly to FAP⁺ cancer-associated fibroblasts and, in some cancers, to FAP⁺ tumor cells; in addition, they indirectly irradiate FAP^- cells in tumor tissue via cross-fire and bystander effects. Here, we discuss the potential to improve FAP-radioligand therapy through interfering with DNA damage repair, immunotherapy, and co-targeting cancer-associated fibroblasts. As the molecular and cellular effects of FAP-radioligands on the tumor and its microenvironment have not been investigated yet, we call for future research to close this gap in knowledge, which prevents the development of more effective FAP-radioligand therapies.

Correlation of 68Ga-FAPi-46 PET Biodistribution with FAP Expression by Immunohistochemistry in Patients with Solid Cancers: Interim Analysis of a Prospective Translational Exploratory Study

Fibroblast activation protein (FAP)-expressing cancer-associated fibroblasts confer treatment resistance and promote metastasis and immunosuppression. Because FAP is overexpressed in many cancers, radiolabeled molecules targeting FAP are studied for their use as pancancer theranostic agents. This study aimed to establish the spectrum of FAP expression across various cancers by immunohistochemistry and to explore whether 68Ga FAP inhibitor (FAPi)-46 PET biodistribution faithfully reflects FAP expression from resected cancer and non-cancer specimens. Methods: We conducted a FAP expression screening using immunohistochemistry on a pancancer human tissue microarray (141 patients, 14 different types of cancer) and an interim analysis of a prospective exploratory imaging trial in cancer patients. Volunteer patients underwent 1 whole-body 68Ga-FAPi-46 PET/CT scan and, subsequently, surgical resection of their primary tumor or metastasis. 68Ga-FAPi-46 PET SUVmax and SUVmean was correlated with FAP immunohistochemistry score in cancer and tumor-adjacent non-cancer tissues for each patient. Results: FAP was expressed across all 14 cancer types on tissue microarray with variable intensity and frequency, ranging from 25% to 100% (mean, 76.6% ± 25.3%). Strong FAP expression was observed in 50%-100% of cancers of the bile duct, bladder, colon, esophagus, stomach, lung, oropharynx, ovary, and pancreas. Fifteen patients with various cancer types (colorectal [n = 4], head and neck [n = 3], pancreas [n = 2], breast [n = 2], stomach [n = 1], esophagus [n = 2], and uterus [n = 1]) underwent surgery after their 68Ga-FAPi-46 PET/CT scan within a mean interval of 16.1 ± 14.4 d. 68Ga-FAPi-46 SUVs and immunohistochemistry scores were higher in cancer than in tumor-adjacent non-cancer tissue: mean SUVmax 7.7 versus 1.6 (P < 0.001), mean SUVmean 6.2 versus 1.0 (P < 0.001), and mean FAP immunohistochemistry score 2.8 versus 0.9 (P < 0.001). FAP immunohistochemistry scores strongly correlated with 68Ga-FAPi 46 SUVmax and SUVmean: r = 0.781 (95% CI, 0.376-0.936; P < 0.001) and r = 0.783 (95% CI, 0.379-0.936; P < 0.001), respectively. Conclusion: In this interim analysis of a prospective exploratory imaging trial, 68Ga-FAPi-46 PET biodistribution across multiple cancers strongly correlated with FAP tissue expression. These findings support further exploration of FAPi PET as a pancancer imaging biomarker for FAP expression and as a stratification tool for FAP-targeted therapies.

Mechanisms of Resistance to Prostate-Specific Membrane Antigen-Targeted Radioligand Therapy in a Mouse Model of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) is effective against prostate cancer (PCa), but all patients relapse eventually. Poor understanding of the underlying resistance mechanisms represents a key barrier to development of more effective RLT. We investigate the proteome and phosphoproteome in a mouse model of PCa to identify signaling adaptations triggered by PSMA RLT. Methods: Therapeutic efficacy of PSMA RLT was assessed by tumor volume measurements, time to progression, and survival in C4-2 or C4-2 TP53^-/- tumor-bearing nonobese diabetic scid γ-mice. Two days after RLT, the proteome and phosphoproteome were analyzed by mass spectrometry. Results: PSMA RLT significantly improved disease control in a dose-dependent manner. Proteome and phosphoproteome datasets revealed activation of genotoxic stress response pathways, including deregulation of DNA damage/replication stress response, TP53, androgen receptor, phosphatidylinositol-3-kinase/AKT, and MYC signaling. C4-2 TP53^-/- tumors were less sensitive to PSMA RLT than were parental counterparts, supporting a role for TP53 in mediating RLT responsiveness. Conclusion: We identified signaling alterations that may mediate resistance to PSMA RLT in a PCa mouse model. Our data enable the development of rational synergistic RLT-combination therapies to improve outcomes for PCa patients.

Abstract LB038: Validation of FAPi PET biodistribution by immunohistochemistry in patients with solid cancers: a prospective exploratory study

Context: Fibroblast activation protein (FAP)-expressing cancer associated fibroblasts (CAFs), a major component of tumor stroma, confer treatment resistance, promote local progression, metastasis and immunosuppression. Because FAP is selectively expressed in the tumor stroma of many cancers, radiolabeled small molecule ligands targeting FAP are being explored for their use as pan-cancer theranostic agents. As a required step for further translation and approval by regulatory agencies FAPi PET imaging must be show to be a reliable biomarker of FAP expression in cancer and non-cancer tissues. The aim of the study is to define the incidence and degree of FAP expression by immunohistochemistry (IHC) across various cancers using Tissue Microarrays (TMAs) and to explore whether gallium-68 FAPi-46 PET image biodistribution faithfully reflects tumor FAP expression from resected tumor and non-tumor specimens. Methods: This was a prospective, exploratory, open-label, single-center imaging trial in cancer patients conducted in 2020. Patients scheduled to undergo surgical resection of the primary tumor and/or metastases were eligible. Patients underwent one whole body 68Ga-FAPI-46 PET/CT scan and subsequently underwent surgical resection of the primary tumor and/or metastasis. The primary endpoint measure was the Correlation of 68Ga-FAPI-46 PET maximum standardized uptake value (SUVmax) with FAP IHC score in cancer and non-cancer tissue. Results: The incidence of FAP expression in TMAs from 14 cancers ranged from 25 to 100% (mean 76.6± 25.3). 15 patients with the following cancer types were included: colorectal (n=4), head and neck (n=3), pancreas (n=2), breast (n=2), stomach (n=1), esophagus (n=2) and uterus (n=1). All 15 patients subsequently underwent surgery after the scan with a mean time interval of16.1 ± 14.4 days (range 1 - 50 days). Tumor resection was not attempted in 2 patients because unresectable. FAPI SUVmax and IHC score were higher in cancer tissue than in normal tissue: mean FAPI SUVmax 7.4±4.6 (range 1.5-15.9) vs 1.6±1.2 (range 0.4-5.1), (p&lt;0.001) and mean FAP IHC score 2.38±0.65 vs 0.54±0.66 (p&lt;0.001), respectively. The FAP IHC score was positively correlated with FAPI SUVmax (pairwise p=0.001, repeated measures correlations r=0.85 (95% CI 0.53-0.95), p&lt;0.001). Conclusions: In this translational study using Tissue Microarrays and a prospective exploratory imaging trial of 15 surgical oncology patients, the FAPI PET uptake and FAP expression per immunohistochemistry were strongly correlated in cancer and non-cancer tissue. The 68Ga-FAPi-46 PET biodistribution across multiple cancers reflects FAP expression as determined by IHC. This translational validation paves the way for large scale prospective trials on the use of 68Ga-FAPi-46 PET/CT as a biomarker and stratification tool for FAP-targeted therapies. Trial Registration: ClinicalTrials.gov Identifier: NCT04147494

Citation Format: Christine E. Mona, Matthias Benz, Firat Hikmat, Tristan G. Grogan, Katharina Lueckerath, Aria A. Razmaria, Rana Riahi, Roger Slavik, Mark D. Grigis, Giuseppe Carlucci, Kimberly A. Kelly, Johannes Czernin, David W. Dawson, Jeremie Calais. Validation of FAPi PET biodistribution by immunohistochemistry in patients with solid cancers: a prospective exploratory study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB038.

Metabolically stable neurotensin analogs exert potent and long-acting analgesia without hypothermia

The endogenous tridecapeptide neurotensin (NT) has emerged as an important inhibitory modulator of pain transmission, exerting its analgesic action through the activation of the G protein-coupled receptors, NTS1 and NTS2. Whereas both NT receptors mediate the analgesic effects of NT, NTS1 activation also produces hypotension and hypothermia, which may represent obstacles for the development of new pain medications. In the present study, we implemented various chemical strategies to improve the metabolic stability of the biologically active fragment NT(8-13) and assessed their NTS1/NTS2 relative binding affinities. We then determined their ability to reduce the nociceptive behaviors in acute, tonic, and chronic pain models and to modulate blood pressure and body temperature. To this end, we synthesized a series of NT(8-13) analogs carrying a reduced amide bond at Lys⁸-Lys⁹ and harboring site-selective modifications with unnatural amino acids, such as silaproline (Sip) and trimethylsilylalanine (TMSAla). Incorporation of Sip and TMSAla respectively in positions 10 and 13 of NT(8-13) combined with the Lys⁸-Lys⁹ reduced amine bond (JMV5296) greatly prolonged the plasma half-life time over 20 h. These modifications also led to a 25-fold peptide selectivity toward NTS2. More importantly, central delivery of JMV5296 was able to induce a strong antinociceptive effect in acute (tail-flick), tonic (formalin), and chronic inflammatory (CFA) pain models without inducing hypothermia. Altogether, these results demonstrate that the chemically-modified NT(8-13) analog JMV5296 exhibits a better therapeutic profile and may thus represent a promising avenue to guide the development of new stable NT agonists and improve pain management.

Immune-Checkpoint Blockade Enhances 225Ac-PSMA617 Efficacy in a Mouse Model of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy (RNT) may increase tumor immunogenicity. We aimed at exploiting this effect by combining RNT with immunotherapy in a mouse model of prostate cancer (PC). Methods: C57BL/6-mice bearing syngeneic RM1-PGLS tumors were treated with ²²⁵Ac-PSMA617, an anti-PD-1 antibody, or both. Therapeutic efficacy was assessed by tumor volume measurements (CT), time to progression (TTP), and survival. Results: PSMA RNT or anti-PD-1 alone tended to prolong TTP (isotype control, 25 d; anti-PD-1, 33.5 d [P = 0.0153]; RNT, 30 d [P = 0.1038]) and survival (control, 28 d; anti-PD-1, 37 d [P = 0.0098]; RNT, 32 d [P = 0.1018]). Combining PSMA RNT and anti-PD-1 significantly improved disease control compared with either monotherapy. TTP was extended to 47.5 d (P ≤ 0.0199 vs. monotherapies), and survival to 51.5 d (P ≤ 0.0251 vs. monotherapies). Conclusion: PSMA RNT and PD-1 blockade synergistically improve therapeutic outcomes in our PC model, supporting the evaluation of RNT and immunotherapy combinations for PC patients.

Abstract 5345: Heterogeneous tumor PSMA expression represents a resistance mechanism to PSMA-targeted radioligand therapy

Radioligand therapy (RLT) with prostate-specific membrane antigen (PSMA)-targeting ligands is effective in ~50% of patients with advanced prostate cancer (PC). Causes for RLT failure are not well understood. The prevalent PSMA heterogeneity in PC might contribute to RLT failure. Here we investigate the relationship between RLT efficacy and PSMA levels per cell, and PSMA heterogeneity. PC cells expressing different levels of PSMA (RM1-PSMA low, medium, high, or RM1-YFP that do not express PSMA), or a mix of PSMA− and PSMA+ cells (RM1-YFP/ RM1-PSMA-high; PC3/ PC3-PIP) at various ratios were subcutaneously injected into NSG mice. Mice were treated with 177Lu- or 225Ac-PSMA617 (RLT) and tumor growth was monitored. In a subset of mice, radioligand uptake (γ-counting), DNA damage and PSMA expression (anti-53BP1 and -PSMA immunohistochemistry, respectively) were quantified in tumors resected 2 days after RLT. Increasing PSMA levels and fractions of PSMA+ cells improved RLT efficacy in both, the RM1 and PC3 model. PSMA expression correlated with radioligand uptake into the tumor and the degree of DNA damage and. Treatment with 225Ac-PSMA617 (vs. 177Lu-PSMA617) improved RLT outcomes and tended to enhance the differences in therapeutic efficacy between experimental groups. Taken together, we demonstrate in mouse models of PC that optimal anti-tumor efficacy of RLT hinges on homogenously high target expression. Although PSMA-RLT is effective even in tumors with low PSMA levels or with a small number of PSMA+ cells, low or heterogeneous PSMA expression might result in undertreatment and selection of treatment resistant clones. Systematic assessment of intra- and inter-lesion PSMA heterogeneity is currently not feasible clinically; however, this issue might be addressed by individual patient dosimetry to optimize safely delivered maximal tumor doses. Clinical studies designed to determine intra- and inter-lesion PSMA heterogeneity and to optimize PSMA-RLT for each patient are highly warranted.

Citation Format: Katharina Lueckerath, Kyle Current, Catherine Meyer, Clara Magyar, Christine E. Mona, Joel Almajano, Roger Slavik, Andreea D. Stuparu, Chloe Cheng, David Dawson, Caius Radu, Johannes Czernin. Heterogeneous tumor PSMA expression represents a resistance mechanism to PSMA-targeted radioligand therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5345.

Cell-penetrating pepducins targeting the neurotensin receptor type 1 relieve pain

Pepducins are cell-penetrating, membrane-tethered lipopeptides designed to target the intracellular region of a G protein-coupled receptor (GPCR) in order to allosterically modulate the receptor's signaling output. In this proof-of-concept study, we explored the pain-relief potential of a pepducin series derived from the first intracellular loop of neurotensin receptor type 1 (NTS1), a class A GPCR that mediates many of the effects of the neurotensin (NT) tridecapeptide, including hypothermia, hypotension and analgesia. We used BRET-based biosensors to determine the pepducins' ability to engage G protein signaling pathways associated with NTS1 activation. We observed partial Gα_q and Gα₁₃ activation at a 10 μM concentration, indicating that these pepducins may act as allosteric agonists of NTS1. Additionally, we used surface plasmon resonance (SPR) as a label-free assay to monitor pepducin-induced responses in CHO-K1 cells stably expressing hNTS1. This whole-cell integrated assay enabled us to subdivide our pepducin series into three profile response groups. In order to determine the pepducins' antinociceptive potential, we then screened the series in an acute pain model (tail-flick test) by measuring tail withdrawal latencies to a thermal nociceptive stimulus, following intrathecal (i.t.) pepducin administration (275 nmol/kg). We further evaluated promising pepducins in a tonic pain model (formalin test), as well as in neuropathic (Chronic Constriction Injury) and inflammatory (Complete Freund's Adjuvant) chronic pain models. We report one pepducin, PP-001, that consistently reduced rat nociceptive behaviors, even in chronic pain paradigms. Finally, we designed a TAMRA-tagged version of PP-001 and found by confocal microscopy that the pepducin reached the rat dorsal root ganglia post i.t. injection, thus potentially modulating the activity of NTS1 at this location to produce its analgesic effect. Altogether, these results suggest that NTS1-derived pepducins may represent a promising strategy in pain-relief.

Isoquinoline thiosemicarbazone displays potent anticancer activity with in vivo efficacy against aggressive leukemias

A potent class of isoquinoline-based α-N-heterocyclic carboxaldehyde thiosemicarbazone (HCT) compounds has been rediscovered; based upon this scaffold, three series of antiproliferative agents were synthesized through iterative rounds of methylation and fluorination modifications, with anticancer activities being potentiated by physiologically relevant levels of copper. The lead compound, HCT-13, was highly potent against a panel of pancreatic, small cell lung carcinoma, prostate cancer, and leukemia models, with IC₅₀ values in the low-to-mid nanomolar range. Density functional theory (DFT) calculations showed that fluorination at the 6-position of HCT-13 was beneficial for ligand-copper complex formation, stability, and ease of metal-center reduction. Through a chemical genomics screen, we identify DNA damage response/replication stress response (DDR/RSR) pathways, specifically those mediated by ataxia-telangiectasia and Rad3-related protein kinase (ATR), as potential compensatory mechanism(s) of action following HCT-13 treatment. We further show that the cytotoxicity of HCT-13 is copper-dependent, that it promotes mitochondrial electron transport chain (mtETC) dysfunction, induces production of reactive oxygen species (ROS), and selectively depletes guanosine nucleotide pools. Lastly, we identify metabolic hallmarks for therapeutic target stratification and demonstrate the in vivo efficacy of HCT-13 against aggressive models of acute leukemias in mice.

Targeted alpha therapy in a systemic mouse model of prostate cancer - a feasibility study

225Ac-PSMA-617 targeted-therapy has demonstrated efficacy in 75-85% of patients; however, responses are not durable. We aimed to establish translatable mouse models of disseminated prostate cancer (PCa) to evaluate effectiveness of 225Ac-PSMA-617 at various disease stages. Methods: C4-2, C4-2B, or 22Rv1 cells were injected into the left ventricle of male NSG mice. Disease progression was monitored using bioluminescence imaging (BLI). For treatment, mice were injected with 40 kBq 225Ac-PSMA-617 at one (early treatment cohort) or three weeks (late treatment cohort) post-inoculation. Treatment efficacy was monitored by BLI of whole-body tumor burden. Mice were sacrificed based on body conditioning score. Results: C4-2 cells yielded metastases in liver, lungs, spleen, stomach, bones, and brain - achieving a clinically relevant model of widespread metastatic disease. The disease burden in the early treatment cohort was stable over 27 weeks in 5/9 mice and progressive in 4/9 mice. These mice were sacrificed due to brain metastases. Median survival of the late treatment cohort was superior to controls (13 vs. 7 weeks; p<0.0001) but inferior to that in the early treatment cohort (13 vs. 27 weeks; p<0.001). Late cohort mice succumbed to extensive liver involvement. The 22Rv1 and C4-2B systemic models were not used for treatment due to high kidney metastatic burden or low take rate, respectively. Conclusion: C4-2 cells reproduced metastatic cancer spread most relevantly. Early treatment with 225Ac-PSMA-617 prevented liver metastases and led to significant survival benefit. Late treatment improved survival without reducing tumor burden in the liver, the main site of metastasis. The current findings suggest that early 225Ac-PSMA-617 intervention is more efficacious in the setting of widespread metastatic PCa.

Investigating PSMA-Targeted Radioligand Therapy Efficacy as a Function of Cellular PSMA Levels and Intratumoral PSMA Heterogeneity

PURPOSE

Prostate-specific membrane antigen (PSMA) targeting radioligands deliver radiation to PSMA-expressing cells. However, the relationship between PSMA levels and intralesion heterogeneity of PSMA expression, and cytotoxic radiation by radioligand therapy (RLT) is unknown. Here we investigate RLT efficacy as function of PSMA levels/cell, and the fraction of PSMA⁺ cells in a tumor.

EXPERIMENTAL DESIGN

RM1 cells expressing different levels of PSMA (PSMA^-, PSMA⁺, PSMA⁺⁺, PSMA⁺⁺⁺; study 1) or a mix of PSMA⁺ and PSMA^- RM1 (study 2, 4) or PC-3/PC-3-PIP (study 3) cells at various ratios were injected into mice. Mice received ¹⁷⁷Lu- (studies 1-3) or ²²⁵Ac- (study 4) PSMA617. Tumor growth was monitored. Two days post-RLT, tumors were resected in a subset of mice. Radioligand uptake and DNA damage were quantified.

RESULTS

¹⁷⁷Lu-PSMA617 efficacy increased with increasing PSMA levels (study 1) and fractions of PSMA positive cells (studies 2, 3) in both, the RM1 and PC-3-PIP models. In tumors resected 2 days post-RLT, PSMA expression correlated with ¹⁷⁷Lu-PSMA617 uptake and the degree of DNA damage. Compared with ¹⁷⁷Lu-PSMA617, ²²⁵Ac-PSMA617 improved overall antitumor effectiveness and tended to enhance the differences in therapeutic efficacy between experimental groups.

CONCLUSIONS

In the current models, both the degree of PSMA expression and the fraction of PSMA⁺ cells correlate with ¹⁷⁷Lu-/²²⁵Ac-PSMA617 tumor uptake and DNA damage, and thus, RLT efficacy. Low or heterogeneous PSMA expression represents a resistance mechanism to RLT.See related commentary by Ravi Kumar and Hofman, p. 2774.

Preclinical evaluation of PSMA expression in response to androgen receptor blockade for theranostics in prostate cancer

Background

Prostate-specific membrane antigen (PSMA)-directed radioligand therapy (RLT) is a promising yet not curative approach in castration-resistant (CR) prostate cancer (PC). Rational combination therapies may improve treatment efficacy. Here, we explored the effect of androgen receptor blockade (ARB) on PSMA expression visualized by PET and its potential additive effect when combined with ¹⁷⁷Lu-PSMA RLT in a mouse model of prostate cancer.

Methods

Mice bearing human CRPC (C4-2 cells) xenografts were treated with 10 mg/kg enzalutamide (ENZ), with 50 mg/kg bicalutamide (BIC), or vehicle (control) for 21 days. PSMA expression was evaluated by ⁶⁸Ga-PSMA11 PET/CT and quantified by flow cytometry of tumor fine needle aspirations before treatment and on days 23, 29, 34, and 39 post-therapy induction. For the RLT combination approach, mice bearing C4-2 tumors were treated with 10 mg/kg ENZ or vehicle for 21 days before receiving either 15 MBq (84 GBq/μmol) ¹⁷⁷Lu-PSMA617 or vehicle. DNA damage was assessed as phospho-γH2A.X foci in tumor biopsies. Reduction of tumor volume on CT and survival were used as study endpoints.

Results

Tumor growth was delayed by ARB while ⁶⁸Ga-PSMA11 uptake increased up to 2.3-fold over time when compared to controls. ABR-induced upregulation of PSMA expression was confirmed by flow cytometry. Phospho-γH2A.X levels increased 1.8- and 3.4-fold at 48 h in response to single treatment ENZ or RLT and ENZ+RLT, respectively. Despite significantly greater DNA damage and persistent increase of PSMA expression at the time of RLT, no additional tumor growth retardation was observed in the ENZ+RLT group (vs. RLT only, p = 0.372 at day 81). Median survival did not improve significantly when ENZ was combined with RLT.

Conclusion

ARB-mediated increases in PSMA expression in PC xenografts were evident by ⁶⁸Ga-PSMA11 PET imaging and flow cytometry. ¹⁷⁷Lu-PSMA617 effectively decreased C4-2 tumor size. However, while pre-treatment with ARB increased DNA damage significantly, it did not result in synergistic effects when combined with RLT.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0451-z) contains supplementary material, which is available to authorized users.

Detection Threshold and Reproducibility of 68Ga-PSMA11 PET/CT in a Mouse Model of Prostate Cancer

To improve prostate-specific membrane antigen (PSMA)-targeted theranostic approaches, robust murine models of prostate cancer are needed. However, important characteristics of preclinical PSMA imaging-that is, the reproducibility of the imaging signal and the relationship between quantitative cell surface PSMA expression and lesion detectability with small-animal PET/CT-have not been defined yet. Methods: Murine prostate cancer RM1 sublines (ras myc transformed cells of C57BL/6 prostate origin) expressing varying levels of human PSMA were injected into the shoulder of C57BL/6 mice on day 0. ⁶⁸Ga-PSMA11 PET/CT was performed on days 7 and 8 and interpreted by 2 masked readers to determine interday and interreader reproducibility. PSMA expression was quantified on days 7 and 8 by flow cytometry of fine-needle aspiration tumor biopsy samples. Cell surface PSMA expression was correlated with PET signal. The threshold for PET positivity was based on the clinical Prostate Cancer Molecular Imaging Standardized Evaluation (PROMISE) criteria. Results: The maximum and average percentages of injected ⁶⁸Ga-PSMA11 activity per gram of tissue (%IA/g) correlated nearly perfectly as determined by 2 independent readers and on 2 separate days (intraclass correlation coefficient, 1.00/0.89 and 0.95/0.88, respectively). The number of PSMA molecules per cell increased from the RM1-yellow fluorescent protein subline (PSMA^-; 2,000/cell) to the RM1-low subline (PSMA⁺; 17,000/cell), the RM1-medium subline (PSMA⁺⁺; 22,000/cell), and the RM1-PGLS subline (PSMA-positive, green fluorescent protein-positive, and luciferase-positive; PSMA⁺⁺⁺; 45,000/cell). Expression levels correlated with the visual positivity rate on ⁶⁸Ga-PSMA11 PET and with the PSMA PET %IA/g. The PSMA threshold for PET positivity was approximately 20,000 per cell. Signal correlation was close at lower PSMA levels (RM1-low to RM1-medium; 10-23 %IA/g) but was lost at higher PSMA levels (RM1-medium to RM1-PGLS; 23-27 %IA/g). Conclusion: The in vivo relationship between ⁶⁸Ga-PSMA11 PET/CT and PSMA expression level in a murine model of prostate cancer was robust for lower cell surface PSMA expression levels (≤22,000/cell). Thus, preclinical ⁶⁸Ga-PSMA11 PET/CT can be used as an imaging biomarker to test PSMA-targeted interventions in murine models.

Design, synthesis, and biological evaluation of CXCR4 ligands

An amino functionalized analog of the CXCR4 ligand IT1t is of higher affinity and inverse agonistic potency on the CXCR4-CAM receptor N119S than IT1t.