Preclinical Comparison of the Blood-brain barrier Permeability of Osimertinib with Other EGFR TKIs.

PURPOSE: Osimertinib is a potent and selective EGFR tyrosine kinase inhibitor (EGFR-TKI) of both sensitizing and T790M resistance mutations. To treat metastatic brain disease, blood-brain barrier (BBB) permeability is considered desirable for increasing clinical efficacy. EXPERIMENTAL DESIGN: We examined the level of brain penetration for 16 irreversible and reversible EGFR-TKIs using multiple in vitro and in vivo BBB preclinical models. RESULTS: In vitro osimertinib was the weakest substrate for human BBB efflux transporters (efflux ratio 3.2). In vivo rat free brain to free plasma ratios (Kpuu) show osimertinib has the most BBB penetrance (0.21), compared with the other TKIs (Kpuu ≤ 0.12). PET imaging in Cynomolgus macaques demonstrated osimertinib was the only TKI among those tested to achieve significant brain penetrance (C max %ID 1.5, brain/blood Kp 2.6). Desorption electrospray ionization mass spectroscopy images of brains from mouse PC9 macrometastases models showed osimertinib readily distributes across both healthy brain and tumor tissue. Comparison of osimertinib with the poorly BBB penetrant afatinib in a mouse PC9 model of subclinical brain metastases showed only osimertinib has a significant effect on rate of brain tumor growth. CONCLUSIONS: These preclinical studies indicate that osimertinib can achieve significant exposure in the brain compared with the other EGFR-TKIs tested and supports the ongoing clinical evaluation of osimertinib for the treatment of EGFR-mutant brain metastasis. This work also demonstrates the link between low in vitro transporter efflux ratios and increased brain penetrance in vivo supporting the use of in vitro transporter assays as an early screen in drug discovery.

First Radiolabeling of a Ganglioside with a Positron Emitting Radionuclide: In Vivo PET Demonstrates Low Exposure of Radiofluorinated GM1 in Non-human Primate Brain.

Gangliosides are biologically important glycolipids widely distributed in vertebrate cells. An important member of the ganglioside family is the monosialylganglioside GM1, which has been suggested as a potential therapeutic for Parkinson's disease. In the current study, a late-stage radiofluorination protocol was developed, in which fluorine-18 was introduced by substitution of a terminal tosyl group in the fatty acid backbone of GM1. The radiofluorination procedure was remarkably simple and furnished the radiofluorinated ganglioside, [18F]F-GM1, in sufficient quantity and quality without protection of the glycosyl moiety. A positron emission tomography measurement in cynomolgus monkey revealed high uptake of [18F]F-GM1 in heart, bone marrow, and lungs but low (<0.4% of injected dose) distribution to the brain. Thus, choosing administration route of GM1 for therapy of central nervous system disorders poses further challenges. The present study demonstrates the importance of application of positron emission tomography microdosing studies in guiding early clinical drug development.

Synthesis and Preclinical Evaluation of 6-[18F]Fluorine-α-methyl-l-tryptophan, a Novel PET Tracer for Measuring Tryptophan Uptake.

The positron emission tomography (PET) radioligand α-[11C]methyl-l-tryptophan ([11C]AMT) has been used to assess tryptophan metabolism in cancer, epilepsy, migraine, and autism. Despite its extensive application, the utility of this tracer is currently hampered by the short half-life of the radionuclide used for its labeling (11C, t1/2 = 20.4 min). We herein report the design, synthesis, radiolabeling, and initial in vivo evaluation of a fluorine-18 (18F, t1/2 = 109.7 min) labeled analogue that is fluorinated in the 6-position of the aromatic ring ([18F]6-F-AMTr). In a head-to-head comparison between [18F]6-F-AMTr and [11C]AMT in mice using PET, peak brain radioactivity, regional brain distribution, and kinetic profiles were similar between the two tracers. [18F]6-F-AMTr was however not a substrate for IDO1 or TPH as determined in in vitro enzymatic assays. The brain uptake of the tracer is thus more likely related to LAT1 transport over the blood-brain barrier than metabolism along the serotonin or kynurenine pathways.

Automated synthesis and PET evaluation of both enantiomers of [¹8F]FMISO.

INTRODUCTION: [(18)F]FMISO, the widely used positron emission tomography (PET) hypoxia tracer, is a chiral compound clinically used as a racemic mixture. The purpose of this study was to synthesize the individual (R)- and the (S)- enantiomers of [(18)F]FMISO and compare their PET imaging characteristics. METHODS: The radiosynthesis of enantiopure (R)- and (S)-[(18)F]FMISO was based on Co(salen) (N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminocobalt)-mediated opening of enantiopure epoxides with [(18)F]HF. The uptake and clearance of the individual [(18)F]FMISO antipodes were investigated using micro-PET/CT imaging performed on mice bearing FaDu tumors. Image-derived biodistribution was obtained from micro-PET/CT scans performed at 1 and 3 hours post injection (p.i.). In addition, the uptake patterns of each enantiomer were observed using two-hour dynamic micro-PET/CT scans, and the time-activity curves from different organs were compared. RESULTS: The individual (R)- and (S)-[(18)F]FMISO enantiomers were synthesized in one step with high enantiomeric excess (ee)>99% and radiochemical purity>97% using custom-made automation module. The dynamic micro-PET/CT scanning revealed a faster initial uptake of the (R)-[(18)F]FMISO enantiomer in tumor and muscle tissues, however the difference became progressively smaller with time. The tumor-to-muscle (T/M) and tumor-to-liver (T/L) ratios remained nearly identical for the (R)- and (S)-forms at all time points. The micro-PET/CT imaging at 1 and 3 hours p.i. did not show any significant enantioselective tissue uptake. CONCLUSIONS: Although the (R)-enantiomer of [(18)F]FMISO demonstrated a somewhat faster initial tumor and muscle uptake no significant enantioselective tissue uptake was observed at later time points. The T/M- and T/L- ratios for the (R)- and (S)-forms were the same within the experimental error at all times. Therefore, the use of enantiopure [(18)F]FMISO is unlikely to present any practical clinical benefit for PET imaging.