Brain tumor is a general term for tumors occurring within the brain, and is classified into primary and metastatic according to the origin of the cancer. Primary brain tumors start in the brain and metastatic brain tumors are metastasized from other organs to the brain. Brain is the most important human organ because it is responsible for five senses, motor function, cognitive function, memory and decision making. It is protected with two tough defense mechanisms: skull as the physical barrier, and blood-brain barrier as the chemical barrier to block toxic compounds from entering the brain. Incidence rate for brain tumor is low compared with other cancers such as lung, liver, or stomach cancer; but once it is developed, it is extremely difficult to treat due to two defense mechanisms mentioned above. For this reason, it is critical to early diagnosis of brain tumor and its recurrence. According to statistical result (Cancer Res Treat. 2018; 50(2): 303-0316), there were 1,776 new cases, 1,266 mortalities, and 10,349 total number of patients for brain tumor in Korea. 5-year survival rate from 1993 to 2015 in all cancers increased from 41.2% to 70.7% (29.5% increase) while 5-year survival rate for brain tumor only increased from 38.5% to 40.7% (2.2% increase). It is fair to say that the lack of diagnostic PET radiopharmaceuticals for early detection of brain tumor is responsible for the low survival rate increase in brain tumor. Even though 18F-FDG is considered a gold standard of radiopharmaceuticals in cancer diagnosis, it cannot be used for brain tumor detection because of naturally high glucose uptake in the brain. Currently, brain tumor is diagnosed using MRI or CT. But with these imaging techniques, it is difficult to find a small and recurrent brain tumor. It also brings the risk of false-positive in diagnosis because it is difficult to distinguish post-surgery inflammation/tissue damage from brain tumor. In addition, high grade malignant brain tumors infiltrate into surrounding normal brain tissues, and MRI and CT cannot detect the infiltrated area. Therefore, at molecular level, brain tumor target PET radiopharmaceuticals are required for accurate brain tumor diagnosis.
Diagnostic PET Radiopharmaceutical for Brain Tumor
Tumor cells has uncontrolled cell division and require a great amount of nutrients. 3’-18F-fluoro-3’-deoxy-L-thymidine (18F-FLT) is a derivative of thymidine required for DNA replication, characterized by relatively high uptake in tumors with active cell division. However, the amount of influxed 18F-FLT into brain detected is not quite large, and it results in unclear PET images (low sensitivity). While high grade (grade 3 or grade 4) malignant brain tumors have destroyed BBB to allow more 18F-FLT influx to the brain and result in clearer PET images; , low grade (grade 1 or grade 2) brain tumors are difficult to be detected with 18F-FLT. Tumors also require amino acids for endless cell division, so radiolabeled amino acids have been studied as tumor diagnostic agents. A representative amino acid compound, 11C-labeled 11C-Cystein has been developed as a PET radiopharmaceutical for brain tumor.. However, the short half-life (20 minutes) of C-11 becomes an obstacle for commercial development. In addition, non-specific binding of 11C-Cystein to inflammatory tissues is the demerits of this compound. In contrast, tyrosine is more specific to tumor than inflammation, so 18F-labeled various tyrosine compounds have been developed. For example, (O-(2-18F-fluroethyl)tyrosine, or 18F-FET, has high brain influx and brain tumor uptake, and differ from 18F-FLT, it can detect brain tumors from low to high grade brain tumor. However, the image quality of 18F-FLT is poor due to high nonspecific normal tissue uptake. 18F-FMT18F-FMT refers to tyrosine combined with 18F-fluoromethyl, which has lower lipophilicity than (O-(3-18F-fluropropyl)tyrosine, 18F-FPT and 18F-FET. 18F-FMT has relatively low non-specific binding and fast clearance in normal tissues, thus enable to produce clear tumor images. In addition, the stereochemically D-form FMT, which is not a naturally occurring L-form, has a low influx into the cell by the L-amino acid transporters. Therefore, there is low uptake into normal tissues compared to relatively high uptake into tumor cells. In In-vivo tumor model mice experiment, tumor to blood ratio of 18F-D-FMT was twice as high as 18F-L-FMT at 60 minutes after the injection. This ratio value is higher than that of the most studied 18F-FET compound to date, and suggests that 18F-D-FMT can specifically target to the tumor with extremely low normal tissue uptake.
A conventional manufacturing of 18F-FMT consists of three steps: first, synthesis of 18F-labeled prosthetic group; second, reaction with tyrosine with 18F-labeled prosthetic group; finally deprotection reaction. However, this method cannot be used for commercial purposes due to its long synthesis and low manufacturing yield. We have developed a triazolium salt leaving group and applied it to 18F-FMT precursor synthesis, which enabled high yields of 18F-D-FMT by shorter synthesis.
Phase 0 clinical study was performed in Korea Cancer Center Hospital for patients with primary brain tumor, metastatic brain tumor, and healthy controls. By studying pharmacokinetic characteristic from healthy individuals’ whole-body PET scans, it was confirmed that 18F-FMT was excreted quickly through kidneys after circulation. Most of the drug was removed via urinary tract within 5 minutes after the injection. Since there was no 18F-D-FMT accumulation in liver, lungs, intestine, and other major organs in the body, whole-body absorbed dose was considered very safe (13.2 mSv/MBq). By looking at 18F-D-FMT PET images of brain tumor patient, tumor and normal brain tissue was clearly distinguishable.