Preclinical evaluation of 68Ga- and 177Lu-labeled integrin αvβ6-targeting radiotheranostic peptides

Rationale: The integrin α_vβ₆, an epithelial-specific cell surface receptor, is overexpressed on numerous malignancies, including the highly lethal pancreatic ductal adenocarcinomas (PDAC). Here, we developed and tested a novel α_vβ₆-targeting peptide, DOTA-5G (1) radiolabeled with gallium-68 for positron emission tomography/computed tomography (PET/CT) imaging, and lutetium-177 for treatment. With the goal to develop a radiotheranostic, further modifications were made for increased circulation time, renal recycling, and tumor uptake, yielding DOTA-ABM-5G (2). Methods: Peptides 1 and 2 were synthesized on solid phase and their affinity for αvβ6 assessed by ELISA. The peptides were radiolabeled with gallium-68 and lutetium-177. In vitro cell binding, internalization, and efflux of ⁶⁸Ga-1 and ¹⁷⁷Lu-2 were evaluated in α_vβ₆ (+) BxPC-3 human pancreatic cancer cells. PET/CT imaging of ⁶⁸Ga-1 and ⁶⁸Ga-2 was performed in female nu/nu mice bearing subcutaneous BxPC-3 tumors. Biodistribution was performed for ⁶⁸Ga-1 (1 and 2 h p.i.), ⁶⁸Ga-2 (2 and 4 h p.i.), and ¹⁷⁷Lu-1 and ¹⁷⁷Lu-2 (1, 24, 48, and 72 h p.i.). The ¹⁷⁷Lu-2 biodistribution data were extrapolated for human dosimetry data estimates using OLINDA/EXM 1.1. Therapeutic efficacy of ¹⁷⁷Lu-2 was evaluated in mice bearing BxPC-3 tumors. Results: Peptides 1 and 2 demonstrated high affinity for α_vβ₆ by ELISA. ⁶⁸Ga-1, ⁶⁸Ga-2, ¹⁷⁷Lu-1 and ¹⁷⁷Lu-2 were synthesized in high radiochemical purity (RCP). Rapid in vitro binding and internalization of ⁶⁸Ga-1 and ¹⁷⁷Lu-2 were observed in BxPC-3 cells. PET/CT imaging and biodistribution studies demonstrated uptake in BxPC-3 tumors. Introduction of the ABM in ¹⁷⁷Lu-2 resulted in a 5-fold increase in tumor uptake and retention over time. Based on the extended dosimetry data the dose-limiting organ for ¹⁷⁷Lu-2 are the kidneys. Treatment with ¹⁷⁷Lu-2 prolonged survival. Conclusion: ⁶⁸Ga-1 and ¹⁷⁷Lu-2 demonstrated high affinity for the integrin α_vβ₆ both in vitro and in vivo, were rapidly internalized into BxPC-3 cells, and were stable in mouse and human serum. Both radiotracers showed favorable pharmacokinetics in pre-clinical studies with predominantly renal excretion and good tumor-to-normal tissue ratios. Favorable human dosimetry data suggest the potential of ¹⁷⁷Lu-2 as a treatment for PDAC.

Multiparametric cardiac18F-FDG PET in humans: pilot comparison of FDG delivery rate with82Rb myocardial blood flow

Myocardial blood flow (MBF) and flow reserve are usually quantified in the clinic with positron emission tomography (PET) using a perfusion-specific radiotracer (e.g.⁸²Rb-chloride). However, the clinical accessibility of existing perfusion tracers remains limited. Meanwhile,¹⁸F-fluorodeoxyglucose (FDG) is a commonly used radiotracer for PET metabolic imaging without similar limitations. In this paper, we explore the potential of¹⁸F-FDG for myocardial perfusion imaging by comparing the myocardial FDG delivery rateK₁with MBF as determined by dynamic⁸²Rb PET in fourteen human subjects with heart disease. Two sets of FDGK₁were derived from one-hour dynamic FDG scans. One was the original FDGK₁estimates and the other was the correspondingK₁values that were linearly normalized for blood glucose levels. A generalized Renkin-Crone model was used to fit FDGK₁with Rb MBF, which then allowed for a nonlinear extraction fraction correction for converting FDGK₁to MBF. The linear correlation between FDG-derived MBF and Rb MBF was moderate (r= 0.79) before the glucose normalization and became much improved (r> 0.9) after glucose normalization. The extraction fraction of FDG was also similar to that of Rb-chloride in the myocardium. The results from this pilot study suggest that dynamic cardiac FDG-PET with tracer kinetic modeling has the potential to provide MBF in addition to its conventional use for metabolic imaging.

Multiparametric Cardiac 18F-FDG PET in Humans: Kinetic Model Selection and Identifiability Analysis

Cardiac ¹⁸F-FDG PET has been used in clinics to assess myocardial glucose metabolism. Its ability for imaging myocardial glucose transport, however, has rarely been exploited in clinics. Using the dynamic FDG-PET scans of ten patients with coronary artery disease, we investigate in this paper appropriate dynamic scan and kinetic modeling protocols for efficient quantification of myocardial glucose transport. Three kinetic models and the effect of scan duration were evaluated by using statistical fit quality, assessing the impact on kinetic quantification, and analyzing the practical identifiability. The results show that the kinetic model selection depends on the scan duration. The reversible two-tissue model was needed for a one-hour dynamic scan. The irreversible two-tissue model was optimal for a scan duration of around 10-15 minutes. If the scan duration was shortened to 2-3 minutes, a one-tissue model was the most appropriate. For global quantification of myocardial glucose transport, we demonstrated that an early dynamic scan with a duration of 10-15 minutes and irreversible kinetic modeling was comparable to the full one-hour scan with reversible kinetic modeling. Myocardial glucose transport quantification provides an additional physiological parameter on top of the existing assessment of glucose metabolism and has the potential to enable single tracer multiparametric imaging in the myocardium.

αvβ6-Targeted Molecular PET/CT Imaging of the Lungs After SARS-CoV-2 Infection

The true impact and long-term damage to organs such as the lungs after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection remain to be determined. Noninvasive molecularly targeted imaging may play a critical role in aiding visualization and understanding of the systemic damage. We have identified α_vβ₆ as a molecular target; an epithelium-specific cell surface receptor that is low or undetectable in healthy adult epithelium but upregulated in select injured tissues, including fibrotic lung. Herein we report the first human PET/CT images using the integrin α_vβ₆-binding peptide (¹⁸F-α_vβ₆-BP) in a patient 2 mo after the acute phase of infection. Minimal uptake of ¹⁸F-α_vβ₆-BP was noted in normal lung parenchyma, with uptake being elevated in areas corresponding to opacities on CT. This case suggests that ¹⁸F-α_vβ₆-BP PET/CT is a promising noninvasive approach to identify the presence and potentially monitor the persistence and progression of lung damage.

Preclinical Development and First-in-Human Imaging of the Integrin αvβ6 with [18F]αvβ6-Binding Peptide in Metastatic Carcinoma

PURPOSE

The study was undertaken to develop and evaluate the potential of an integrin αvβ6-binding peptide (αvβ6-BP) for noninvasive imaging of a diverse range of malignancies with PET.

EXPERIMENTAL DESIGN

The peptide αvβ6-BP was prepared on solid phase and radiolabeled with 4-[18F]fluorobenzoic acid. In vitro testing included ELISA, serum stability, and cell binding studies using paired αvβ6-expressing and αvβ6-null cell lines. In vivo evaluation (PET/CT, biodistribution, and autoradiography) was performed in a mouse model bearing the same paired αvβ6-expressing and αvβ6-null cell xenografts. A first-in-human PET/CT imaging study was performed in patients with metastatic lung, colon, breast, or pancreatic cancer.

RESULTS

[18F]αvβ6-BP displayed excellent affinity and selectivity for the integrin αvβ6 in vitro [IC50(αvβ6) = 1.2 nmol/L vs IC50(αvβ3) >10 μmol/L] in addition to rapid target-specific cell binding and internalization (72.5% ± 0.9% binding and 52.5% ± 1.8%, respectively). Favorable tumor affinity and selectivity were retained in the mouse model and excretion of unbound [18F]αvβ6-BP was rapid, primarily via the kidneys. In patients, [18F]αvβ6-BP was well tolerated without noticeable adverse side effects. PET images showed significant uptake of [18F]αvβ6-BP in both the primary lesion and metastases, including metastasis to brain, bone, liver, and lung.

CONCLUSIONS

The clinical impact of [18F]αvβ6-BP PET imaging demonstrated in this first-in-human study is immediate for a broad spectrum of malignancies.

OPTIMA: a phase II dose and volume de-escalation trial for human papillomavirus-positive oropharyngeal cancer

BACKGROUND

Patients with HPV+ oropharyngeal squamous cell carcinoma were assigned to dose and volume de-escalated radiotherapy (RT) or chemoradiotherapy (CRT) based on response to induction chemotherapy in an effort to limit treatment-related toxicity while preserving efficacy.

PATIENTS AND METHODS

Patients were classified as low-risk (≤T3, ≤N2B, ≤10 pack-year history) or high-risk (T4 or ≥N2C or >10 PYH). After three cycles of carboplatin/nab-paclitaxel, response was assessed using Response Evaluation Criteria in Solid Tumors 1.1. Low-risk patients with ≥50% response received 50 Gray (Gy) RT (RT50) while low-risk patients with 30%-50% response or high-risk patients with ≥50% response received 45 Gy CRT (CRT45). Patients with lesser response received standard-of-care 75 Gy CRT (CRT75). RT/CRT was limited to the first echelon of uninvolved nodes. The primary end point was 2-year progression-free survival compared with a historic control of 85%. Secondary end points included overall survival and toxicity.

RESULTS

Sixty-two patients (28 low risk/34 high risk) were enrolled. Of low-risk patients, 71% received RT50 while 21% received CRT45. Of high-risk patients, 71% received CRT45. With a median follow-up of 29 months, 2-year PFS and OS were 95% and 100% for low-risk patients and 94% and 97% for high-risk patients, respectively. The overall 2-year PFS was 94.5% and within the 11% noninferiority margin for the historic control. Grade 3+ mucositis occurred in 30%, 63%, and 91% of the RT50, CRT45, and CRT75 groups, respectively (P = 0.004). Rates of any PEG-tube use were 0%, 31%, and 82% for RT50, CRT45, and CRT75 groups, respectively (P < 0.0001).

CONCLUSIONS

Induction chemotherapy with response and risk-stratified dose and volume de-escalated RT/CRT for HPV+ OPSCC is associated with favorable oncologic outcomes and reduced acute and chronic toxicity. Further evaluation of induction-based de-escalation in large multicenter studies is justified.

CLINICAL TRIAL REGISTRATION

Clinical trials.gov identifier: NCT02258659.

Abstract 3045: Pilot study of kernel-based dynamic (KBD) FDG-PET in patients with borderline resectable pancreatic cancer

Introduction: Patients (pts) with borderline resectable (BR) pancreatic cancer (PC) derive a clear survival benefit when margin-negative resection can be achieved after neoadjuvant treatment (tx). As such, reliable imaging is desperately needed to identify those patients with the best chance of surgical benefit. Current imaging with CT/MRI and standard 18F-fludeoxyglucose (FDG) PET lacks sensitivity and specificity to resolve residual tumor contact with the large blood vessels of the vascular groove. Dynamic PET with kernel-based image reconstruction developed by our group has potential to overcome limits of standard static PET by robust parametric imaging of radiotracer kinetics. KBD FDG-PET provides the FDG kinetic parameter K1 for tumor perfusion and Ki for tumor glucose utilization. A lower Ki/K1 ratio indicates a better perfused, less metabolically active tumor and is derived by our method without need for separate perfusion CT/MRI. Methods: Pts with PC staged as BR as defined by consensus guidelines were eligible. Pts were assessed by CA 19-9 and CT/MRI along with KBD FDG-PET both pre-tx and post-tx. KBD FDG PET was performed by IV bolus of 10 mCi of FDG followed by 60-min PET data acquisition. A 3-compartment model with 5 micro kinetic parameters K1, k2, k3, k4, and fv was used where K1 (mL/min/mL) denotes the rate constant of FDG transport from plasma to tissue, k2 (1/min) the transport rate from tissue to plasma, k3 (1/min) the rate of FDG conversion to FDG 6-phosphate, k4 (1/min) the rate of dephosphorylation, and fv the fractional blood volume. Ki was calculated from micro parameters by the formula K1*k3/(k2+k3). Parametric images of these kinetic parameters were obtained by voxel-wise implementation of kinetic modeling. Results: 4 pts were enrolled, 3 pts had pre- and post-tx KBD FDG-PET, and 2 pts were resected. In the 3 evaluable pts, an overall 62% decrease in tumor glucose utilization was observed (mean pre-tx Ki=.03, [range .013-.045]; mean post-tx Ki=.011 [range .004-.016]. Mean pre-tx K1=.37 (range .26-.50) and mean post-tx K1=.38 (range .31-.46). Delta Ki was -17%, -47%, and -91% and delta Ki/K1 was -9%, -28%, and -95% in pts 1, 2, 3 respectively. Pt 3 had the highest pre-tx Ki and the lowest pre-tx K1, but had a &gt;90% decrease in Ki and Ki/K1 post-tx, indicating a significant decrease in metabolic activity and concomitant increase in perfusion. Despite persistent superior mesenteric vein (SMV) abutment on standard post-tx imaging, margin-negative resection was achieved. In contrast, pt 1 had the lowest pre-tx Ki and highest pre-tx K1, but had a &lt;20% decrease in Ki and Ki/K1 post-tx. Like Pt 3, Pt 1 also had SMV contact on standard post-tx imaging, but in contrast, experienced margin-positive resection. Conclusions: KBD FDG-PET is a promising modification of standard static PET that provides useful kinetic parameters for evaluation of PC resectability.

Citation Format: John P. Schwerkoske, Guobao Wang, Kit W. Tam, Jasmine C. Huynh, Heather H. Hunt, Michael L. Rusnak, Cameron C. Foster, Michael T. Corwin, Karen E. Matsukuma, Dorina Gui, May T. Cho, Richard J. Bold, Ramsey D. Badawi, Edward J. Kim. Pilot study of kernel-based dynamic (KBD) FDG-PET in patients with borderline resectable pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3045.