Development of novel PET probes targeting phosphatidylinositol 3-kinase (PI3K) in tumors

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.

11C-Labeled Pictilisib (GDC-0941) as a Molecular Tracer Targeting Phosphatidylinositol 3-Kinase (PI3K) for Breast Cancer Imaging

Pictilisib (GDC-0941) is an inhibitor of phosphatidylinositol 3-kinase (PI3K), part of a signaling cascade involved in breast cancer development. The purpose of this study was to evaluate the pharmacokinetics of pictilisib noninvasively by radiolabeling it with ¹¹C and to assess the usability of the resulting [¹¹C]-pictilisib as a positron-emission tomography (PET) tracer to screen for pictilisib-sensitive tumors. In this study, pictilisib was radiolabeled with [¹¹C]-methyl iodide to obtain ¹¹C-methylated pictilisib ([¹¹C]-pictilisib) using an automated synthesis module with a high radiolabeling yield. Considerably higher uptake ratios were observed in MCF-7 (PIK3CA mutation, pictilisib-sensitive) cells than those in MDA-MB-231 (PIK3CA wild-type, pictilisib-insensitive) cells at all evaluated time points, indicating good in vitro binding of [¹¹C]-pictilisib. Dynamic micro-PET scans in mice and biodistribution results showed that [¹¹C]-pictilisib was mainly excreted via the hepatobiliary tract into the intestines. MCF-7 xenografts could be clearly visualized on the static micro-PET scans, while MDA-MB-231 tumors could not. Biodistribution results of two xenograft models showed significantly higher uptake and tumor-to-muscle ratios in the MCF-7 xenografts than those in MDA-MB-231 xenografts, exhibiting high in vivo targeting specificity. In conclusion, [¹¹C]-pictilisib was first successfully prepared, and it exhibited good potential to identify pictilisib-sensitive tumors noninvasively, which may have a great impact in the treatment of cancers with an overactive PI3K/Akt/mTOR signal pathway. However, the high activity in hepatobiliary system and intestines needs to be addressed.

Preclinical Evaluation of a Fluorine-18 (18F)-Labeled Phosphatidylinositol 3-Kinase Inhibitor for Breast Cancer Imaging

Breast cancer is one of the commonest malignancies in women, especially in middle-aged and elderly women. Abnormal activation of the phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKt/mTOR) pathway has been found to be involved in breast cancer proliferation. Pictilisib (GDC-0941) is a potent inhibitor of PI3K with high affinity and is undergoing phase 2 clinical trials. In this study, we aimed to develop a noninvasive PI3K radiotracer to help determine the mechanism of the PI3K/AKt/mTOR pathway to aid in diagnosis. We designed a new ¹⁸F-radiolabeled radiotracer based on the structure of pictilisib, to evaluate noninvasively abnormal activation of the PI3K/AKT/mTOR pathway. To increase the water solubility, and to decrease hepatobiliary and gastrointestinal uptake of the tracer, pictilisib was modified with triethylene glycol di(p-toluenesulfonate) (TsO-PEG₃-OTs) to obtain TsO-PEG₃-GDC-0941 as the precursor for ¹⁸F labeling. A nonradiolabeled reference compound [¹⁹F]-PEG₃-GDC-0941 was also prepared. Breast cancer cell lines, MCF-7 and MDA-MB-231, were used as high- and low-expression PI3K models, respectively. PET imaging and ex vivo biodistribution assays of [¹⁸F]-PEG₃-GDC-0941 in MCF-7 and MDA-MB-231 xenografts were also performed, and the results were compared. The precursor compound and reference standard compound were successfully synthesized and identified using NMR and mass spectroscopy. The ¹⁸F radiolabeling was achieved with a high yield (61 ± 1%) at a high molar activity (2100 ± 100 MBq/mg). MicroPET images and biodistribution studies showed a higher uptake of the radiotracer in MCF-7 tumors than in MDA-MB-231 tumors (7.56 ± 1.01%ID/g vs 4.07 ± 0.68%ID/g, 1 h postinjection). Additionally, the MCF-7 tumor uptake was significantly decreased when a blocking dose of GDC-0941 was coinjected, indicating high specificity. The liver was found to be the major excretory organ with 5.82 ± 0.88%ID/g at 30 min postinjection for MCF-7 mice. This radiotracer holds great potential for patient screening, diagnosis, and therapy prediction of PI3K-related diseases.

Identification of novel PI3Kδ inhibitors by docking, ADMET prediction and molecular dynamics simulations

BACKGROUND

Phosphoinositide-3-kinase Delta (PI3Kδ) plays a key role in B-cell signal transduction and inhibition of PI3Kδ is confirmed to have clinical benefit in certain types of activation of B-cell malignancies. Virtual screening techniques have been used to discover new molecules for developing novel PI3Kδ inhibitors with little side effects.

METHOD

Computer aided drug design method were used to rapidly screen optimal PI3Kδ inhibitors from the Asinex database. Virtual screening based molecular docking was performed to find novel and potential lead compound targeting PI3Kδ, at first. Subsequently, drug likeness studies were carried out on the retrieved hits to evaluate and analyze their drug like properties such as absorption, distribution, metabolism, excretion, and toxicity (ADMET) for toxicity prediction. Three least toxic compounds were selected for the molecular dynamics (MD) simulations for 30 ns in order to validate its stability inside the active site of PI3Kδ receptor.

RESULTS

Based on the present in silico analysis, two molecules have been identified which occupied the same binding pocket confirming the selection of active site. ASN 16296138 (Glide score: -12.175 kcal/mol, cdocker binding energy: -42.975 kcal/mol and ΔG_bind value: -90.457 kcal/mol) and BAS 00227397 (Glide score: -10.988 kcal/mol, cdocker binding energy: -39.3376 kcal/mol and ΔG_bind value: -81.953 kcal/mol) showed docking affinities comparatively much stronger than those of already reported known inhibitors against PI3Kδ. These two ligand's behaviors also showed consistency during the simulation of protein-ligand complexes for 30000 ps respectively, which is indicative of its stability in the receptor pocket.

CONCLUSION

Compound ASN 16296138 and BAS 00227397 are potential candidates for experimental validation of biological activity against PI3Kδ in future drug discovery studies. This study smoothes the path for the development of novel leads with improved binding properties, high drug likeness, and low toxicity to humans for the treatment of cancer.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.