Process for synthesis of 11C-labeled methyl iodide and apparatus

Abstract: In a process for the synthesis of ¹¹C-labeled methyl iodide which comprises producing ¹¹CO₂ by the irradiation of proton beam, supplying ¹¹CO₂ gas to a collector of ¹¹CO₂ gas, bubbling ¹¹CO₂ gas into a reducing agent solution to reduce ¹¹CO₂, removing the reducing agent solution by the evaporation, synthesizing ¹¹CH₃I from the reduced product of ¹¹CO₂, and recovering ¹¹CH₃I by distillation, the termination point of the ¹¹CO₂ gas supply, the termination point of the bubbling, the termination of the evaporation of the reducing agent solution or the termination point of the distillation of ¹¹CH₃I is decided by a radiation sensor provided in the vicinity of the reaction vessel or the transfer tube connecting the target box wherein ¹¹CO₂ gas is generated or a temperature sensor provide in the exhaust tube of the reaction vessel. The synthesizing time is shortened, and the yield of ¹¹CH₃I is improved by improving the utilization of ¹¹CO₂ gas and the recovery of ¹¹CH₃I of which the half-life is very short.

Description: This invention relates to a process for the synthesis of a labeled compound, namely 11 C-labeled methyl iodide, used for the positron emission tomography (PET) system which is one of the imaging diagnostic technologies and apparatuses therefor. A labeled compound used for the PET system is methyl iodide labeled with 11 C which was synthesized using a synthetic apparatus as shown in Figure 7. In this figure, the numeral 1 indicates a target gas cylinder which stores a mixed gas for the target, and the target gas cylinder 1 is connected with a target box 4 in which 11 CO 2 gas is producd by a transfer tube 3 through an electromagnetic valve 2. The target box 4 is connected with a collecting coil 8 by a transfer tube 7 through electromagnetic valves 5,6. A helium gas cylinder 9 is also connected with the collecting coil 8 by transfer tubes 11,7 through electromagnetic valves 10,6. The collecting coil 8 is put in a cooling vessel 12, and the outlet of the collecting coil 8 is connected with a reaction vessel 14 by a transfer tube 13. The reaction vessel 14 is further connected with a syringe 16 containing hydroiodic acid by a transfer tube 18 through an electromagnetic valve 17 as well as an exhaust tube 15. When methyl iodide labeled with 11 C is synthesized using the above synthetic apparatus, the mixed gas for the target is filled into the target box 4 from the target gas cylinder 1 by opening the valve 2. Then, proton beam 19 supplied from a cyclotron (not shown) is irradiated for a fixed time to produce 11 CO 2 gas through a nuclear reaction ( 14 N(p·d) 11 C). Subsequently, the target gas containing 11 CO 2 is delivered to the collecting coil 8 cooled in the cooling vessel 12 through the valves 5,6, and 11 CO 2 gas is collected. After the collection is finished, the collecting coil 8 is heated to deliver 11 CO 2 gas to the reaction vessel 14 by supplying helium gas from the gas cylinder 9 through the valves 10,6. In the reaction vessel 14, 11 CO 2 gas is reduced by bubbling it into a reducing agent solution. Then, the reducing agent solution is evaporated by heating the reaction vessel, and discharged through the exhaust tube 15. Hydroiodic acid (HI) is introduced into the reaction vessel 14 by operating the syringe 16, and methyl iodide labeled with 11 C ( 11 CH 3 I) is synthesized. Thereafter, 11 CH 3 I is recovered by heating the reaction vessel 14 again to distill it. In the above process of synthesizing methyl iodide, respective times for terminating bubbling, the distillation of the reducing agent solution and the supply of 11 CO 2 were decided from an average time necessary for these processes which was empirically decided by adding an excess time for the security. These times were inputted into an apparatus having a time control such as a microcomputer or a sequencer as a set point, and the finishment of these processes was recognized. In the above conventional process, since respective termination points of the bubbling, the distillation of the reducing agent solution and the supply of 11 CO 2 were decided empirically, these points were set considerably longer than the minimum time due to the variations of bubbling time, the distillation time of the reducing agent solution and the supply time of 11 CO 2 caused by a delicate difference of conditions. However, the half life of 11 C is about 20 minutes which is very short, and therefore, quenching of 11 CO 2 and 11 CH 3 I increased by the extension of the set points. As a result, it was difficult that both of the utilization of 11 CO 2 and the recovery of 11 CH 3 I were kept always high. That is, it was impossible to satisfy both of decreasing the risk of failure in the synthesis and increasing the yield of 11 CH 3 I. Moreover, another problem is in a long time from the start of supplying 11 CO 2 to the recovery of 11 CH 3 I. International Journal of Applied Radiation and Isotopes, vol. 36, no. 4, pp 303-306 and International Journal of Applied Radiation and Isotopes, vol. 36, no. 6, pp 469-474 describe a process for the synthesis of 11 CH 3 I. The process comprises 11 CO 2 -producing process, bubbling of 11 CO 2 through THF/LiAlH 4 in a first vessel, evaporation of THF, hydrolysis, and reaction of the resulting 11 CH 3 OH in a second reaction vessel with HI. According to the former document, the THF is evaporated after the level of radioactivity in the vessel reaches maximum. In the process according to the latter reference , the bubbling step and the 11 CO 2 supply step is monitored by a GM tube. The Journal of Nuclear Medicine, vol. 28 (1987), pp 1037-1040 describes a process for the synthesis of 11 CH 3 I, wherein 11 CO 2 is trapped in a vessel containing LiAlH 4 /THF, the THF is removed after the trapping and the LiAlH-complexes hydrolysed by addition of HI. It is the object of the invention to provide a process for the synthesis of 11 C-labeled methyl iodide capable of improving both of the utilization of 11 CO 2 and the recovery of 11 CH 3 I and wherein the 11 C-labeled methyl iodide can be recovered in a short time. Another object of the invention is to provide an apparatus therefor. The above-described object is achieved by a process for the synthesis of 11 C-labeled methyl iodide which comprises a 11 CO 2 -producing process of producing carbon dioxide gas labeled with 11 C, a bubbling process of bubbling 11 CO 2 gas into a reducing agent solution in a reaction vessel, a reducing agent solution-removing process of evaporating the reducing agent solution after the termination of the bubbling, and a 11 CH 3 I synthesis process of synthesising 11 CH 3 I from an intermediate produced by the reduction in the bubbling process in the reaction vessel and deciding the termination of the evaporation in the reducing agent solution-removing process based on the variation of the temperature in an exhaust tube for discharging the vapour of the reducing agent solution connected with the reaction vessel. The present invention provides further an apparatus for the synthesis of 11 C-labeled methyl iodide which comprises a target box in which 11 CO 2 gas is produced, a reaction vessel in which 11 CH 3 I is synthesised from 11 CO 2 gas, a transfer tube connecting the target box with the reaction vessel and a temperature sensor for detecting the temperature of an exhaust tube for discharging the vapour of the reducing agent solution reducing 11 CO 2 gas. The process according to the present invention further comprises a 11 CH 3 I distillation process of distilling 11 CH 3 I synthesised in the 11 CH 3 I synthesis process, and deciding the termination of the bubbling in the bubbling process and the termination of the distillation in the 11 CH 3 I distillation process based on the variation of the radiation emitted from 11 C in the reaction vessel. The process according to the present invention further comprises a 11 CO 2 -supplying process of supplying 11 CO 2 gas produced in the 11 CO 2 producing process to a reaction vessel and deciding the termination of the supply of 11 CO 2 gas in the 11 CO 2 -supplying process based on the variation of the radiation emitted from 11 C on the introducing side of 11 CO 2 gas. The apparatus according to the present invention further comprises a radiation sensor which detects the radiation emitted from 11 C provided in the vicinity of the reaction vessel. The apparatus according to the present invention further comprises a radiation-detecting part provided at the transfer tube, and a radiation sensor which detects the radiation emitted from 11 C provided in the vicinity of the part at which radiation is detected. Figure 1 is a block diagram illustrating an apparatus as used in comparative Example 1. Figure 2 is a chart indicating an output curve of a radiation sensor obtained by using the apparatus of Figure 1. Figure 3 is a block diagram illustrating the apparatus embodying the invention. Figure 4 is a chart indicating an output curve of a temperature sensor obtained by using the apparatus of Figure 3. Figure 5 is a block diagram illustrating the apparatus as used in Comparative Example 3. Figure 6 is a chart indicating an output curve of a radiation sensor obtained by using the apparatus of Figure 5. Figure 7 is a block diagram of a conventional apparatus. In the process of the invention, the termination point of the bubbling is decided by utilizing the increase of radiation due to the introduction of 11 CO 2 into the reaction vessel by bubbling, and the termination point of the distillation of 11 CH 3 I is decided by utilizing the decrease of radiation due to the discharge of 11 CH 3 I from the reaction vessel by the distillation. That is, when the radiation becomes a maximum, it is judged that the bubbling is terminated. When the radiation becomes a minimum, it is judged that the distillation of 11 CH 3 I is terminated. The termination point of the evaporation of the reducing agent solution is decided by detecting the temperature in an exhaust tube for discharging the vapor of the reducing agent solution connected with the reaction vessel. In this process, the temperature variation due to the evaporation of the reducing agent solution is utilized, i.e. the temperature rises together with the start of the evaporation and drops by the termination of the evaporation. When the temperature in the exhaust tube becomes a minimum, it is judged that the evaporation of the reducing agent solution is terminated. The temperature variation accompanied by the evaporation of the reducing agent solution is preferably detected by measuring the temperature in the reaction vessel. However, since the corrosive action of hydroiodic acid placed in the reaction vessel is very strong, it is difficult to place a temperature sensor in the reaction vessel. Therefore, the temperature sensor is attached to the exhaust tube, and the temperature in the exhaust tube is detected. The termination point of the supply of 11 CO 2 gas into the reaction vessel is decided by detecting the radiation on the introducing side of 11 CO 2 gas. In this process, it is utilized that the radiation is great during supplying 11 CO 2 gas, but it is small when the supply is terminated. When the radiation is sharply decreased, it is judged that the supply of 11 CO 2 gas is terminated. The detecting point of the radiation may be any point between the target box and the reaction vessel. The above maximum point and the minimum point of the radiation emitted from the reaction vessel, the minimum point of the temperature in the exhaust tube, and the sharp decrease of the radiation on the introducing side of 11 CO 2 gas can be decided by visual observation of various displays indicating the output of the radiation sensor. Alternatively, the output of the radiation sensor is automatically decided by inputting it into a controller such as a microcomputer, and the next work is automatically started. EXAMPLES Comparative Example 1 An apparatus for the synthesis of 11 C-labeled methyl iodide is shown in Figure 1. The apparatus is composed of a target gas cylinder 1 which stores a mixed gas for the target, and the target gas cylinder 1 is connected with a target box 4 in which 11 CO 2 gas is produced by a transfer tube 3 through an electromagnetic valve 2. The target box 4 is connected with a collecting coil 8 by a transfer tube 7 through electromagnetic valves 5,6. A helium gas cylinder 9 is also connected with the collecting coil 8 by transfer tubes 11,7 through electromagnetic valves 10,6. The collecting coil 8 is put in a cooling vessel 12, and the outlet of the collecting coil 8 is connected with a reaction vessel 14 by a transfer tube 13. The reaction vessel 14 is further connected with a syringe 16 containing hydroiodic acid by a transfer tube 18 through an electromagnetic valve 17 as well as an exhaust tube 15. A radiation sensor 21 is disposed adjacent to the reaction vessel, and the radiation sensor 21 is connected with a presonal computer 23 through an A/D converter 22. 11 C-labeled methyl iodide was produced using the above apparatus. As the mixed gas for the target, highly pure N 2 gas (99.999 %) or a mixed gas of 90 % of N 2 gas and 10 % of O 2 gas was used, and filled into the target box 4 from the target gas cylinder 1 by opening the valve 2 at a pressure of 5-30 kg/cm 2 . Then, proton beam 19 supplied from a cyclotron (not shown) was irradiated for 10-60 minutes to produce 11 CO 2 gas through a nuclear reaction ( 14 N(p·d) 11 C). Subsequently, the target gas containing 11 CO 2 was delivered to the collecting coil 8 colled at -186°C by the cooling vessel 12 through the valves 5,6, and 11 CO 2 gas was collected. After the collection was finished, the collecting coil 8 was heated to deliver 11 CO 2 gas to the reaction vessel 14 by supplying helium gas from the gas cylinder 9 through the valves 10,6. In the reaction vessel, 11 CO 2 gas was reduced by bubbling it into 50-30 µl of (a saturated solution of LiAlH 4 and THF) solution as the reducing agent solution. The radiation emitted from the reaction vessel 14 was measured by the radiation sensor 21, and the electric signal outputted therefrom was converted to a digital signal by the A/D converter 22. The digital signal was inputted in the microcomputer 23. The microcomputer 23 treated the digital signal data to indicate the strength of the radiation on a display as a voltage value, and decided the termination point of the bubbling. The output voltage curve x was shown in Figure 2. When the bubbling was started, the output voltage rose from the start point a of the bubbling. The output voltage rise was stopped at the termination point b of the bubbling. The termination point b was decided by the microcomputer 23, and the computer 23 instructed to start heating of the reaction vessel 14. The reducing agent solution was evaporated by the heating, and discharged through the exhaust tube 15. Thus, a complex of 11 CO 2 and LiAlH 4 remained in the reaction vessel 14 as the intermediate. 1 ml of 55-58 % hydroiodic acid solution was introduced into the reaction vessel 14 by operating the syringe 16 to synthesize methyl iodide labeled with 11 C. The output voltage was almost constant from the termination of the bubbling to the 11 CH 3 I synthesis. Then, the reaction vessel 14 was heated again, and 11 CH 3 I was recovered by distillation. The output voltage dropped from the start point c of the distillation, and the output voltage drop was stopped at the termination point d of the distillation. The microcomputer 23 decided the termination point of the distillation, and instructed to stop the recovery. The synthetic time of 11 CH 3 I was 7-8 minutes. The utilization of 11 CO 2 gas was 98 %, and the recovery of 11 CH 3 I was 70 %. Whereas, in the conventional process conducted using the above apparatus except the radiation sensor 21, the synthetic time was 15-20 minutes. The utilization of 11 CO 2 gas was 88 %, and the recovery of 11 CH 3 I was 50 %. Example 2 An apparatus for the synthesis of 11 C-labeled methyl iodide is shown in Figure 3. This apparatus is the same as employed in Comparative Example 1 except that the radiation sensor 21 is not provided but a temperature sensor 24 is provided. In detail, a branch tube 25 is connected with the exhaust tube 15, and a temperature detecting element 26 is inserted into the exhaust tube 15. The temperature detecting element 26 is connected with a microcomputer 28 through an A/D converter 27. 11 C-labeled methyl iodide was produced using the above apparatus similar to Comparative Example 1. The bubbling time was set the same 4-5 minutes as the conventional process, and the distillation time was set the same 4-5 minutes as the conventional process. The temperature in the exhaust tube 15 was measured by the temperature sensor 24, and the electric signal outputted therefrom was converted to a digital signal by the A/D converter 27. The digital signal was inputted in the microcomputer 28. The microcomputer 28 treated the digital signal data to indicate the temperature on a display as a voltage value, and decided the termination point of the evaporation of the reducing agent solution. The output voltage curve y was shown in Figure 4. After the termination of the bubbling, when the evaporation of the reducing agent solution was started, the output voltage gradually rose from the start point e of the evaporation. Then, the output voltage gradually dropped, and it was a minimum at the termination point f of the evaporation. The termination point f was decided by the microcomputer 28, and the computer 28 instructed to work the syringe 16. The hydroiodic acid was injected into the reaction vessel 14, and 11 CH 3 I was synthesized. The synthetic time of 11 CH 3 I was 7-8 minutes. The utilization of 11 CO 2 gas was 98 %, and the recovery of 11 CH 3 I was 70 %. Whereas, in the conventional process conducted using the above apparatus except the temperature sensor 24, the synthetic time was 15-20 minutes. The utilization of 11 CO 2 gas was 98 %, and the recovery of 11 CH 3 I was 50 %. Comparative Example 3 An apparatus for the synthesis of 11 C-labeled methyl iodide is shown in Figure 5. This apparatus is the same as Comparative Example 1 except that the radiation sensor 21 is provided near the detecting coil 30 of the transfer tube 7 connecting the target box 4 with the collecting coil 8 instead of the reaction vessel 14. 11 C-labeled methyl iodide was produced using the above apparatus similar to Comparative Example 1. The bubbling time was set the same 4-5 minutes as the conventional process, and the distillation time was set the same 4-5 minutes as the conventional process. The evaporation time of the reducing agent solution was set the same 2-3 minutes as the conventional process. The output voltage curve z was shown in Figure 6. When the supply of 11 CO 2 gas was started, the output voltage rose from the start point g of the supply. While the supply of 11 CO 2 gas was almost constant, the output voltage curve z was almost constant. When the supply of 11 CO 2 gas was terminated, the output voltage dropped. The computer 23 decided the termination point of the supply, and instructed to start the collection of 11 CO 2 gas by the collecting coil 8. The synthetic time of 11 CH 3 I was 7-8 minutes. The utilization of 11 CO 2 gas was 98 %, and the recovery of 11 CH 3 I was 70 %. Whereas, in the conventional process conducted using the above apparatus except the radiation sensor 21, the synthetic time was 15-20 minutes. The utilization of 11 CO 2 gas was 98 %, and the recovery of 11 CH 3 I was 50 %.

Claims: A process for the synthesis of 11 C-labeled methyl iodide which comprises a 11 CO 2 -producing process of producing carbon dioxide gas labeled with 11 C, a bubbling process of bubbling 11 CO 2 gas into a reducing agent solution in a reaction vessel, a reducing agent solution-removing process of evaporating the reducing agent solution after the termination of the bubbling, and a 11 CH 3 I synthesis process of synthesising 11 CH 3 I from an intermediate produced by the reduction in the bubbling process in the reaction vessel and deciding the termination of the evaporation in the reducing agent solution-removing process based on the variation of the temperature in an exhaust tube for discharging the vapour of the reducing agent solution connected with the reaction vessel. An apparatus for the synthesis of 11 C-labeled methyl iodide which comprises a target box in which 11 CO 2 gas is produced, a reaction vessel in which 11 CH 3 I is synthesised from 11 CO 2 gas, a transfer tube connecting the target box with the reaction vessel and a temperature sensor for detecting the temperature of an exhaust tube for discharging the vapour of the reducing agent solution reducing 11 CO 2 gas. The process according to claim 1 which further comprises a 11 CH 3 I distillation process of distilling 11 CH 3 I synthesised in the 11 CH 3 I synthesis process, and deciding the termination of the bubbling in the bubbling process and the termination of the distillation in the 11 CH 3 I distillation process based on the variation of the radiation emitted from 11 C in the reaction vessel. The apparatus according to claim 2 which further comprises a radiation sensor which detects the radiation emitted from 11 C provided in the vicinity of the reaction vessel. The process according to claim 1 which further comprises a 11 CO 2 -supplying process of supplying 11 CO 2 gas produced in the 11 CO 2 producing process to a reaction vessel and deciding the termination of the supply of 11 CO 2 gas in the 11 CO 2 -supplying process based on the variation of the radiation emitted from 11 C on the introducing side of 11 CO 2 gas. The apparatus according to claim 2 which further comprises a part at which radiation is detected provided at the transfer tube, and a radiation sensor which detects the radiation emitted from 11 C provided in the vicinity of the part at which radiation is detected. The process of claim 1 wherein the reducing agent is LiAlH 4 . The process of claim 1 wherein the 11 CH 3 I is synthesised by adding hydroiodic acid.

Inventors: FUJISAWA YOSHIKI, NAKAGAWA HIDEYUKI, TAKAHASHI NAOKO, YAMAZAKI SHIGEKI, FUJISAWA, YOSHIKI, NAKAGAWA, HIDEYUKI, TAKAHASHI, NAOKO, YAMAZAKI, SHIGEKI, Fujisawa, Yoshiki, c/o NKK CORPORATION, Nakagawa, Hideyuki, c/o NKK CORPORATION, TAKAHASHI, NAOKO, ROOM 404 KASA MADERA, Yamazaki, Shigeki, c/o NKK CORPORATION
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1991
Main classification: C07B59
IPC codes: C07B59