An Assessment of Early Response to Targeted Therapy via Molecular Imaging: A Pilot Study of 3'-deoxy-3'[(18)F]-Fluorothymidine Positron Emission Tomography 18F-FLT PET/CT in Prostate Adenocarcinoma

Deciphering Tumor Response: The Role of Fluoro-18-d-Glucose Uptake in Evaluating Targeted Therapies with Tyrosine Kinase Inhibitors

Background/Objectives: The inhibitory effects of tyrosine kinase inhibitors (TKIs) on glucose uptake through their binding to human glucose transporter-1 (GLUT-1) have been well documented. Thus, our research aimed to explore the potential impact of various TKIs of GLUT-1 on the standard [18F]FDG-PET monitoring of tumor response in patients. Methods: To achieve this, we conducted an analysis on three patients who were undergoing treatment with different TKIs and harbored actionable alterations. Alongside the assessment of FDG data (including SUVmax, total lesion glycolysis (TLG), and metabolic tumor volume (MTV)), we also examined the changes in tumor sizes through follow-up [18F]FDG-PET/CT imaging. Notably, our patients harbored alterations in BRAFV600, RET, and c-KIT and exhibited positive responses to the targeted treatment. Results: Our analysis revealed that FDG data derived from SUVmax, TLG, and MTV offered quantifiable outcomes that were consistent with the measurements of tumor size. Conclusions: These findings lend support to the notion that the inhibition of GLUT-1, as a consequence of treatment efficacy, could be indirectly gauged through [18F] FDG-PET/CT imaging in cancer patients undergoing TKI therapy.

Tracking Docetaxel-Induced Cellular Proliferation Changes in Prostate Tumor-Bearing Mice with 18F-FMAU PET

OBJECTIVES

The aim of this exploratory preclinical study was to evaluate the efficacy of 18F-FMAU PET in quantitatively measuring cellular proliferation changes in response to a chemotherapeutic agent in experimental prostate cancer models.

METHODS AND MATERIALS

Docetaxel (DTX) ‒ a standard therapy agent in castrate-resistant metastatic prostate cancer was used as the chemotherapy drug. Athymic male nu/nu mice were inoculated with PC-3 cells in the right flank. After the tumor diameter reached 5 mm, DTX (24 mg/kg) was injected intravenously twice a week, whereas the control group was intravenously administered with saline. The tumor size and body weight were monitored, and longitudinal PET scans were acquired with 18F-FMAU to evaluate tumor cellular proliferation. 18F-FMAU PET scans were performed at 2 hours post-injection of 18F-FMAU on days 0, 11, 18, and 22. Biodistribution studies were carried out after the PET scan on day 22.

RESULTS

Consecutive administrations of DTX were effective in inhibiting PC-3 tumor growth compared to the control group. For PET imaging, PC-3 tumor uptake of 18F-FMAU in the DTX group was increased significantly from 3.09 ± 0.60 %ID/g (day 0) to 5.32 ± 0.37 %ID/g (day 22), whereas the 18F-FMAU tumor update in the control group remained relatively stable on day 0 (2.37 ± 0.51 %ID/g) vs. day 22 (1.83 ± 0.22 %ID/g). The tumor-to-muscle uptake ratio of 18F-FMAU was increased from 2.63 ± 0.20 (day 0) to 5.91 ± 1.1 (day 22) in the DTX group. On day 22, no statistical significance was observed in the tumor-to-muscle uptake ratio of 18F-FMAU in the DTX group vs. the control group. The tumor-to-liver uptake ratio of 18F-FMAU was also similar on day 22 in the DTX group (4.29 ± 0.09) vs. the control group (3.83 ± 0.59).

CONCLUSION

18F-FMAU uptake in implanted PC-3 tumors increases with DTX despite inhibiting tumor growth. Further investigation is needed to decipher the underlying biological mechanism of this apparent flare effect and its relation to the predictability of tumor response to DTX.

Molecular Imaging with 3'-deoxy-3'[(18)F]-Fluorothymidine (18F-FLT) PET/CT for Early Response to Targeted Therapies in Sarcomas: A Pilot Study

Although 3′-deoxy-3′[(18)F]-fluorothymidine (FLT)-positron emission tomography (PET) has been utilized for tumor response assessment to neoadjuvant chemotherapy in soft tissue sarcomas, it has not been exploited for the assessment of early response to systematically targeted therapies. Herein, we investigated the ¹⁸F-FLT PET/CT kinetics in patients with sarcoma who received targeted therapies. Among 15 patients with sarcoma who underwent ¹⁸F-FLT PET/CT, 5 patients (33%) patients were imaged at three time points: At baseline and at 1–15 weeks (MDM2-inhibitor treatment), and 10 patients (67%) were imaged twice: At baseline and at 1–4 weeks (MDM2 inhibitor, n = 5; c-met inhibitor n = 5). The patients with sarcoma had a total of 18 identifiable tumors. Twelve of 15 patients (80%) demonstrated ¹⁸F-FLT concentrations changes early, i.e., at 1–4 weeks. Eight patients responded (53.3%), four patients progressed (26.7%) based on FLT change of more than 10% increase, and three patients (20%) demonstrated no change. ¹⁸F-FLT PET/CT may be used for early response imaging to molecularly targeted therapies in patients with sarcoma. Further larger studies in specific sarcoma sub-types are warranted.

Molecular imaging of metastatic atrial angiosarcoma with positron emission tomography (PET) tracer 3'-deoxy-3'[(18)F]-fluorothymidine, [(18)F]-FLT imaging and early response evaluation

Primary cardiac angiosarcoma, the most common primary cardiac sarcoma has an incidence ranging from 0.001% to 0.028% in autopsy reports with around 200 cases reported in literature. Since a diagnosis of cardiac angiosarcoma portends a poor prognosis, it is vital to ascertain the precise extent of the lesions for follow-up. Imaging with positron emission tomography (PET) tracer 2-deoxy-2-[¹⁸F]-fluoro-D-glucose in cardiac angiosarcoma is challenging as myocardium takes up glucose and delineation of tumour becomes difficult. Cell proliferation rate in normal cardiac muscular tissue is low whereas cardiac tumours display a higher proliferation rate. This aspect could be exploited by use of 3'-deoxy-3'[⁽¹⁸⁾F]-fluorothymidine positron emission tomography (¹⁸F-FLT PET/CT] in cardiac tumours where the cell proliferation could be measured. Herein, we imaged an index case of cardiac angiosarcoma using ¹⁸F-FLT PET/CT and report the findings.