Epigenetic drugs in somatostatin type 2 receptor radionuclide theranostics and radiation transcriptomics in mouse pheochromocytoma models

Abstract

Pheochromocytomas and paragangliomas (PCCs/PGLs) are catecholamine-producing tumors. In inoperable and metastatic cases, somatostatin type 2 receptor (SSTR2) expression allows for peptide receptor radionuclide therapy with [ 177 Lu]Lu-DOTA-TATE. Insufficient receptor levels, however, limit treatment efficacy. This study evaluates whether the epigenetic drugs valproic acid (VPA) and 5-Aza-2'-deoxycytidine (DAC) modulate SSTR2 levels and sensitivity to [ 177 Lu]Lu-DOTA-TATE in two mouse PCC models (MPC and MTT). Methods: Drug-effects on Sstr2/SSTR2 were investigated in terms of promoter methylation, mRNA and protein levels, and radiotracer binding. Radiotracer uptake was measured in subcutaneous allografts in mice using PET and SPECT imaging. Tumor growth and gene expression (RNAseq) were characterized after drug treatments. Results: DAC alone and in combination with VPA increased SSTR2 levels along with radiotracer uptake in vitro in MPC (high-SSTR2) and MTT cells (low-SSTR2). MTT but not MPC allografts responded to DAC and VPA combination with significantly elevated radiotracer uptake, although activity concentrations remained far below those in MPC tumors. In both models, combination of DAC, VPA and [ 177 Lu]Lu-DOTA-TATE was associated with additive effects on tumor growth delay and specific transcriptional responses in gene sets involved in cancer and treatment resistance. Effects of epigenetic drugs were unrelated to CpG island methylation of the Sstr2 promoter. Conclusion:This study demonstrates that SSTR2 induction in mouse pheochromocytoma models has some therapeutic benefit that occurs via yet unknown mechanisms. Transcriptional changes in tumor allografts associated with epigenetic treatment and [ 177 Lu]Lu-DOTA-TATE provide first insights into genetic responses of PCCs/PGLs, potentially useful for developing additional strategies to prevent tumor recurrence.

Metastatic Pheochromocytoma and Paraganglioma: Somatostatin Receptor 2 Expression, Genetics, and Therapeutic Responses

“…These findings are in accordance with clinical reports demonstrating poor tumor detection rates for radiolabeled SSA in EPAS1 -mutated PPGLs with impaired HIF-2α degradation ( 40 ). Furthermore, a recent animal study showed that SSTR2 levels can be stimulated in mouse pheochromocytoma allografts via treatment with epigenetic drugs ( 41 ).…”

Improving susceptibility of neuroendocrine tumors to radionuclide therapies: personalized approaches towards complementary treatments

“…Although SSTR2 upregulating effects have been shown in vitro and in vivo , the underlying mechanism may be unrelated to epigenetic reprogramming 111 . To this end, our group showed that epigenetic modification with valproic acid and 5-aza-2'-deoxycytidine modulates SSTR2 levels and sensitivity to [ 177 Lu]Lu-DOTA-TATE in mouse models for PPGL 53 . Similar to experiences in GEP-NEN models, SSTR2 promoter methylation was also not the cause of SSTR2 induction in PPGL.…”

“…There are two main strategies for enhancing the efficacy of conventional radionuclide therapies: increasing the absorbed radiation dose (dose maximization), or enhancing the tumor's susceptibility to the biophysical and radiobiological effects of ionizing radiation (radiosensitization). The absorbed dose of a target-specific radiopharmaceutical can be improved by (i) optimizing treatment schemes and sequences, e.g., via personalized fractionating and dosing [49]; (ii) minimizing off-target retention, in particular in kidneys, thereby allowing for increased total applied doses of the radiopharmaceutical [16,50,51]; (iii) upregulating molecular targets for radiopharmaceuticals in tumors, e.g., through epigenetic modifiers [52][53][54][55]; (iv) increasing the bound fraction of radiopharmaceuticals in tumors, e.g., using albumin binder conjugates providing increased blood circulation times [56] or receptor antagonists [57,58]; and (v) attaching alternative radionuclides with suitable chemical properties for radiolabeling as well as favorable decay properties providing radiation with high ionization densities such as α particles, Auger electrons and/or high-energy βparticles, and half-lives compatible with the pharmacologic properties of the targeting vectors, e.g. 225 Ac, 213 Bi, 211 At, 212 Pb, 161 Tb, and 67 Cu [59][60][61].…”

Metabolomics in paraganglioma: applications and perspectives from genetics to therapy

“…Additional opportunities may arise with agents targeting DNA or histone methylation (Pang et al, 2019); however, since DNA hypermethylation alone is not sufficient to drive mesenchymal transition, monotherapy may not be effective against metastatic PPGL (Morin et al, 2020). Instead, it may useful as adjuvant therapy supporting PRRT (Ullrich et al, 2023).…”