Production and quality control of [18F]FDOPA
The radiochemical yield of our [18F]FDOPA preparation was 34.15 ± 13.38% and the total synthesis time was 82.7 ± 11.0 min. The radiochemical and enantiomeric purities were 99.8 ± 0.1% and 97.1 ± 0.37%, respectively (n=19).
Effects of ethanol addition at various pH levels
Two kinds of radiochemical impurities were detected by RP-HPLC, based on increases in both the pH and retention time (Figure 1). At 2 hours after the addition of the 8.4% NaHCO3 solution, the radiochemical purity dramatically decreased at pH 6 without EtOH. Over a pH range of 6–8, solutions containing 1-5% EtOH concentrations showed 10% more stability than those without EtOH up to 6 h. For the 5% EtOH solutions, the radiochemical purity at pH 6 and 7 was above 90% up to 6 h and 4 h, respectively. The decreased radiochemical purity in the 1% EtOH solution was comparable between the pH 7 and 8 solutions. Without EtOH however, the rate of decomposition of [18F]FDOPA was greater at pH 8 compared with pH 7.
Interestingly, no free [18F]fluoride was detected by RP-HPLC. Additionally, the retention time of free [18F]fluoride was below 3.0 min under our analysis conditions . Hence, we considered the impurities detected by RP-HPLC at the pH range of 6-8 to be unknown radioactive substances. Furthermore, the enantiomeric purity showed no decrease, and the unknown radiochemical impurities detected RP-HPLC were not detected by chiral-HPLC.
Evaluation of [18F]fluoride content by radio-TLC.
We performed radio-TLC analysis because we could not measure the exact amount of free [18F]fluoride by RP-HPLC. [18F]Fluoride formation was monitored by radio-TLC in the [18F]FDOPA solution before and after neutralization according previously described method . Without neutralization, the [18F]fluoride concentration did not increase until 6 hours. However, as soon as the solution was neutralized (pH=6-7), the [18F]fluoride concentration exponentially increased up to 80% at 25°C. The 5% EtOH solution suppressed this rate of de-[18F]fluorination at the earlier time points at 25°C. After 6 hours however, the [18F]fluoride levels were similar to those in the solution without EtOH. Even at 4°C, the [18F]fluoride concentration showed a slow linear increased up to 20% for 6 hours after neutralization (Figure 2).
Under conditions of 5% EtOH and pH 7, the addition of a buffer solution dramatically suppressed the de-[18F]fluorination of [18F]FDOPA (Figure 3). The preparation containing PBS showed no increase in [18F]fluoride concentration up to the 6 hour timepoint (Figure 4). Both the phosphate and citrate buffers also suppressed the formation of [18F]fluoride, but to a lesser degree as the [18F]fluoride levels still continuously increased up to 5% and 10%, respectively.
Stability of the final [18F]FDOPA formulation
The finally formulated [18F]FDOPA (pH= 6.5) in a 5% EtOH/PBS solution showed good stability up to 6 hours. The initial radiochemical and enantiomeric purities were 100% and 97.8±0.3%, respectively. [18F]fluoride was not detected in the [18F]FDOPA (pH=6.5-7.0) solution at EOS. At 6 hours after EOS, the radiochemical purity was slightly decreased to 99.0±0.5% but the enantiomeric purity was maintained as 97.9±0.2%. The formation of [18F]fluoride was slightly increased to 3.0±1.0% (Figure 3) at this time point.
High radioactivity production, quality control and stability of the neutral pH injectable [18F]FDOPA solution
The radiochemical yield of our newly formulated [18F]FDOPA was 30.4±6.1%. All of the QC parameters for this new preparation met the EP(European Pharmacopeia) criteria. The radiochemical and enantiomeric purities were 100% and 95.8±0.8%, respectively, at EOS. The molar activity was 54.7–15679.7 TBq/mmol at EOS. The ethanol concentration was 4.0 ± 0.38%, and no residual solvents were detected. Although we used EtOH at a 5% concentration in this formulation, it was only detected at 4.0 ± 0.38% in the GC analysis after sterile filtering and delivery through the instrument tubing. The radiochemical and chiral purities at 6 hours after EOS were 99.0 ± 0.46% and 98.54 ± 0.75%, respectively. No additional radiochemical impurities formed during the 6 hour period after preparation at pH 6.5-7.0.