Therapy response monitoring of the early effects of a new BRAF inhibitor on melanoma xenograft in mice: evaluation of (18) F-FDG-PET and (18) F-FLT-PET

Preclinical Evaluation of a Radiolabeled Pan-RAF Inhibitor for RAF-Specific PET/CT Imaging

Abnormalities in the RAS-RAF signaling pathway occur in many solid tumors, leading to aberrant tumor proliferation, invasion, and metastasis. Due to the elusive pharmacology of RAS, RAF inhibitors have become the main targeted therapeutic drugs. Naporafenib (LXH-254) is a high-affinity pan-RAF inhibitor with FDA Fast Track Qualification. We sought to develop an 18F-labeled molecular probe from LXH-254 for PET imaging of tumors overexpressing RAF to noninvasively screen patients for susceptibility to targeted RAF therapy. To reduce the lipid solubility, LXH-254 was designed with triethylene glycol di(p-toluenesulfonate) (TsO-PEG3-OTs) to obtain the precursor (LXH-254-OTs) and a nucleophilic substitution reaction with 18F to obtain the tracer ([18F]F-LXH-254). [18F]F-LXH-254 exhibited good molar activity (7.16 ± 0.81 GBq/μmol), radiochemical purity (>95%), and stability. Micro-PET imaging revealed distinct radioactivity accumulation of [18F]F-LXH-254 in tumors in the imaging groups, whereas in the blocked group, the tumor radioactivity level was consistent with the background tissue, illustrating the affinity and specificity of [18F]F-LXH-254 in targeting RAF. Overall, [18F]F-LXH-254 is a promising radiotracer for screening and diagnosing patients with RAF-related disease and monitoring their treatment. This is the first attempt at using an 18F-labeled RAF-specific radiotracer.

18F-5-FPN: A Specific Probe for Monitoring Photothermal Therapy Response in Malignant Melanoma

Previously, we successfully synthesized a ¹⁸F-labeled positron-emission tomography (PET) tracer, termed ¹⁸F-5-fluoro-N-(2-[diethylamino]ethyl)picolinamide (¹⁸F-5-FPN), with high specificity for melanin. In this study, we sought to investigate the value of ¹⁸F-5-FPN in assessing the response to photothermal therapy (PTT) in melanoma via comparison with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) to reveal an early response, recognize early recurrence, and distinguish the inflammatory response during the treatment. B16F10, inflammatory, and MDA-MB-231 models were subjected to ¹⁸F-FDG PET and ¹⁸F-5-FPN PET static acquisitions. We compared quantitative data to assess the specificity of different agents for different diseases. B16F10 and MDA-MB-231subcutaneous tumor models were irradiated with an 808 nm laser for PTT. Their survival was documented to observe the efficacy of and response to PTT, using ¹⁸F-5-FPN and ¹⁸F-FDG PET. ¹⁸F-5-FPN accumulated in B16F10 cell xenografts only, whereas ¹⁸F-FDG accumulated in all three models. Melanin in B16F10 cell xenografts successfully transformed the optical energy into heat. Hematoxylin and eosin (H&E) staining at 24 h revealed destruction and extensive necrosis of tumor tissue. PTT rapidly inhibited the growth of B16F10 cell xenografts and prolonged the median survival. The mean tumor uptakes of ¹⁸F-5-FPN on day 2 (7.52 ± 3.65 %ID/g) and day 6 (10.22 ± 6.00 %ID/g) were much lower than that before treatment (18.33 ± 4.98 %ID/g, p < 0.01). However, a significant difference in ¹⁸F-FDG uptakes was not found between day 1 after PTT and before treatment. Compared with ¹⁸F-FDG, ¹⁸F-5-FPN PET could estimate PTT efficacy in melanoma, monitor minimal recurrence, and distinguish melanoma from inflammation and other carcinoma types, thanks to its high affinity to melanin. ¹⁸F-5-FPN may provide a new approach for precise and accurate evaluation of response, timely management of therapeutic regimens, and sensitive follow-up.

Preclinical Imaging in Targeted Cancer Therapies

Preclinical imaging with radiolabeled probes can provide noninvasive tools to test the efficacy of targeted agents in tumors harboring specific genetic alterations and to identify imaging parameters that can be used as pharmacodynamics markers in cancer patients. The present review will primarily focus on preclinical imaging studies that can accelerate the clinical approval of targeted agents and promote the development of imaging biomarkers for clinical applications. Since only subgroups of patients may benefit from treatment with targeted anticancer agents, the identification of a patient population expressing the target is of primary importance for the success of clinical trials. Preclinical imaging studies tested the ability of new radiolabeled compounds to recognize mutant, amplified, or overexpressed targets and some of these tracers were transferred to the clinical setting. More common tracers such as ¹⁸F-Fluorothymidine and ¹⁸F-Fluorodeoxyglucose were employed in animal models to test the inhibition of the target and downstream pathways through the evaluation of early changes of proliferation and glucose metabolism allowing the identification of sensitive and resistant tumors. Furthermore, since the majority of patients treated with targeted anticancer agents will invariably develop resistance, preclinical imaging studies were performed to test the efficacy of reversal agents to overcome resistance. These studies provided consistent evidence that imaging with radiolabeled probes can monitor the reversal of drug resistance by newly designed alternative compounds. Finally, despite many difficulties and challenges, preclinical imaging studies targeting the expression of immune checkpoints proved the principle that it is feasible to select patients for immunotherapy based on imaging findings. In conclusion, preclinical imaging can be considered as an integral part of the complex translational process that moves a newly developed targeted agent from laboratory to clinical application intervening in all clinically relevant steps including patient selection, early monitoring of drug effects and reversal of drug resistance.

18F-FDG-PET/CT and diffusion-weighted MRI for monitoring a BRAF and CDK 4/6 inhibitor combination therapy in a murine model of human melanoma

Background

The purpose of the study was to investigate a novel BRAF and CDK 4/6 inhibitor combination therapy in a murine model of BRAF-V600-mutant human melanoma monitored by ¹⁸F–FDG-PET/CT and diffusion-weighted MRI (DW-MRI).

Methods

Human BRAF-V600-mutant melanoma (A375) xenograft-bearing balb/c nude mice (n = 21) were imaged by ¹⁸F–FDG-PET/CT and DW-MRI before (day 0) and after (day 7) a 1-week BRAF and CDK 4/6 inhibitor combination therapy (n = 12; dabrafenib, 20 mg/kg/d; ribociclib, 100 mg/kg/d) or placebo (n = 9). Animals were scanned on a small animal PET after intravenous administration of 20 MBq ¹⁸F–FDG. Tumor glucose uptake was calculated as the tumor-to-liver-ratio (TTL). Unenhanced CT data sets were subsequently acquired for anatomic coregistration. Tumor diffusivity was assessed by DW-MRI using the apparent diffusion coefficient (ADC). Anti-tumor therapy effects were assessed by ex vivo immunohistochemistry for validation purposes (microvascular density – CD31; tumor cell proliferation – Ki-67).

Results

Tumor glucose uptake was significantly suppressed under therapy (∆TTL_Therapy − 1.00 ± 0.53 vs. ∆TTL_Control 0.85 ± 1.21; p < 0.001). In addition, tumor diffusivity was significantly elevated following the BRAF and CDK 4/6 inhibitor combination therapy (∆ADC_Therapy 0.12 ± 0.14 × 10⁻³ mm²/s; ∆ADC_Control − 0.12 ± 0.06 × 10⁻³ mm²/s; p < 0.001). Immunohistochemistry revealed a significant suppression of microvascular density (CD31, 147 ± 48 vs. 287 ± 92; p = 0.001) and proliferation (Ki-67, 3718 ± 998 vs. 5389 ± 1332; p = 0.007) in the therapy compared to the control group.

Conclusion

A novel BRAF and CDK 4/6 inhibitor combination therapy exhibited significant anti-angiogenic and anti-proliferative effects in experimental human melanomas, monitored by ¹⁸F–FDG-PET/CT and DW-MRI.

Vemurafenib plus cobimetinib in unresectable stage IIIc or stage IV melanoma: response monitoring and resistance prediction with positron emission tomography and tumor characteristics (REPOSIT): study protocol of a phase II, open-label, multicenter study

BACKGROUND

In patients with BRAFV600 mutated unresectable stage IIIc or metastatic melanoma, molecular targeted therapy with combined BRAF/MEK-inhibitor vemurafenib plus cobimetinib has shown a significantly improved progression-free survival and overall survival compared to treatment with vemurafenib alone. Nevertheless, the majority of BRAFV600 mutation-positive melanoma patients will eventually develop resistance to treatment. Molecular imaging with 18F-Fluorodeoxyglucose (18F-FDG) PET has been used to monitor response to vemurafenib in some BRAFV600 mutated metastatic melanoma patients, showing a rapid decline of 18F-FDG uptake within 2 weeks following treatment. Furthermore, preliminary results suggest that metabolic alterations might predict the development of resistance to treatment. 18F-Fluoro-3'-deoxy-3'L-fluorothymidine (18F-FLT), a PET-tracer visualizing proliferation, might be more suitable to predict response or resistance to therapy than 18F-FDG.

METHODS

This phase II, open-label, multicenter study evaluates whether metabolic response to treatment with vemurafenib plus cobimetinib in the first 7 weeks as assessed by 18F-FDG/18F-FLT PET can predict progression-free survival and whether early changes in 18F-FDG/18F-FLT can be used for early detection of treatment response compared to standard response assessment with RECISTv1.1 ceCT at 7 weeks. Ninety patients with BRAFV600E/K mutated unresectable stage IIIc/IV melanoma will be included. Prior to and during treatment all patients will undergo 18F-FDG PET/CT and in 25 patients additional 18F-FLT PET/CT is performed. Histopathological tumor characterization is assessed in a subset of 40 patients to unravel mechanisms of resistance. Furthermore, in all patients, blood samples are taken for pharmacokinetic analysis of vemurafenib/cobimetinib. Outcomes are correlated with PET/CT-imaging and therapy response.

DISCUSSION

The results of this study will help in linking PET measured metabolic alterations induced by targeted therapy of BRAFV600 mutated melanoma to molecular changes within the tumor. We will be able to correlate both 18F-FDG and 18F-FLT PET to outcome and decide on the best modality to predict long-term remissions to combined BRAF/MEK-inhibitors. Results coming from this study may help in identifying responders from non-responders early after the initiation of therapy and reveal early development of resistance to vemurafenib/cobimetinib. Furthermore, we believe that the results can be fundamental for further optimizing individual patient treatment.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT02414750. Registered 10 April 2015, retrospectively registered.

18F-FLT and 18F-FDG PET-CT imaging in the evaluation of early therapeutic effects of chemotherapy on Walker 256 tumor-bearing rats

The present study aimed to evaluate the early therapeutic effects of chemotherapy on Walker 256 tumor-bearing Wistar rats via F-18-fluoro-3'-deoxy-3'-L-fluorothymidine (¹⁸F-FLT) and F-18-fluoro-deoxyglucose (¹⁸F-FDG) positron emission tomography-computed tomography (PET-CT) imaging. Walker 256 tumor-bearing Wistar rats were subjected to ¹⁸F-FLT and ¹⁸F-FDG PET-CT imaging prior to and 24 and 48 h after epirubicin chemotherapy. ¹⁸F-FLT and ¹⁸F-FDG uptake [tumor/muscle (T/M)], the percentage of injected dose per gram (% ID/g), and the Ki-67 labeling index (LI-Ki-67) were quantitatively determined for each rat prior to and following epirubicin chemotherapy. The correlation between % ID/g and tumor LI-Ki-67 was analyzed. Both ¹⁸F-FLT and ¹⁸F-FDG tumor uptake decreased significantly at 24 and 48 h after chemotherapy (P<0.01 and P<0.05, respectively). LI-Ki-67 also significantly reduced 24 and 48 h after chemotherapy (P<0.001). Furthermore, ¹⁸F-FLT and ¹⁸F-FDG T/M tumor uptake correlated positively with LI-Ki-67 before and after chemotherapy (r=0.842 and 0.813, respectively). During the early post-chemotherapy stage, ¹⁸F-FLT and ¹⁸F-FDG uptake in Walker 256 tumors reduced significantly, which correlated positively with the tumor cell proliferative activity.

Integration of Mitochondrial Targeting for Molecular Cancer Therapeutics

Mitochondrial metabolism greatly influences cancer cell survival, invasion, metastasis, and resistance to many anticancer drugs. Furthermore, molecular-targeted therapies (e.g., oncogenic kinase inhibitors) create a dependence of surviving cells on mitochondrial metabolism. For these reasons, inhibition of mitochondrial metabolism represents promising therapeutic pathways in cancer. This review provides an overview of mitochondrial metabolism in cancer and discusses the limitations of mitochondrial inhibition for cancer treatment. Finally, we present preclinical evidence that mitochondrial inhibition could be associated with oncogenic “drivers” inhibitors, which may lead to innovative drug combinations for improving the efficacy of molecular-targeted therapy.