The injection of 422 ± 142 MBq of 18F-HX4 in four subjects produced no clinically significant effects on vital signs (blood pressure, temperature, pulse, and EKG) and blood tests during the first 3-4 hour observation period following administration and the follow up visit at 24 hours.
The analysis of plasma samples of 3 subjects showed that the level of un-metabolized 18F-HX4 decreased slowly from 94% at 5 minutes to 82% at 120 minutes after the 18F-HX4 injection. The analysis of the urine samples indicated that the level of un-metabolized 18F-HX4 decreased slowly from 99% at 0-35 minutes to approximately 84% at 105-140 minutes after 18F-HX4 injection. Thus, 18F-HX4 is seen to be essentially intact in the blood over the course of the HX4 PET/CT study.
The main characteristics of the radiotracer uptake are illustrated in PET maximum intensity projection (MIP) images for one of the subjects from the PET scans (). In the first scan, the predominant uptake is seen in the urinary bladder with moderate uptake in liver and kidneys. The gallbladder and large intestine show the 18F-HX4 uptake in later scans. All other organs have the background level of activity. In all subjects, the highest (decay-corrected) uptake of 18F-HX4 was found in the urinary bladder with peak values ranging from 10.9% to 16.4% IA. The second highest uptake was in liver, ranging from 4.3% to 6.4% IA. The %IA decreases significantly between 15 minute and 80 minute time points followed by a gradual decrease to the end of the study (). The radioactivity of 18F-HX4 was excreted primarily via the renal system. By the end of the study (~3.6 hours) ~45% of the injected activity of 18F-HX4 had been excreted in the urine, as determined by assaying urine samples in a well counter. The residence times for the organs are listed in .
Fig. 2 Decay-corrected anterior maximum-intensity projections of PET at 17, 82, 120, 156, and 199 min (from left to right) after injection of 18F-HX4 in a female volunteer. There is rapid clearance of activity in kidneys, liver and bladder. Gallbladder activity (more ...)
Fig. 3 Mean percentage of injected activity and standard deviation (SD) for top 3 organs determined on the basis of 4 18F-HX4 PET emission scans in human volunteers, as a function of time after injection. Rapid clearance of activity is observed in the organs (more ...)
Residence times of source organs for participants injected with 18F-HX4 (n=4, mean ± SD)
The mean effective dose of 18F-HX4 was estimated to be 14 ± 1 μSv/MBq and 27 ± 2 μSv/MBq for 1 hour and 4.8 hour bladder voiding models respectively. The four organs with the highest radiation absorbed doses were urinary bladder wall, gallbladder wall, lower large intestine wall, and kidneys ().
Radiation Dosimetry Estimates for 18F-HX4 in 1 hour and 4.8 hour bladder voiding models, based on human participant data (n=4).
The hypoxia marker 18F-HX4 shows a biodistribution dominated by activity in the bladder and steady renal clearance, with 45% of IA excreted in the 3.6 hours of the study. In the intermediate scans gallbladder activity is noted, and in the last scans intestinal elimination as indicated by intestinal activity can be observed. The relatively wide range of activities seen in the various organs implies wide range of absorbed radiation doses. Apart from urinary bladder, liver, kidneys, gallbladder, and large intestine are seen to have above background level of activity. All other organs displayed near the background level of activity.
Urinary bladder received the highest dose of 18F-HX4 among all the organs. The mean urinary bladder wall dose was 85 ± 10 μGy/MBq and 299 ± 38 μGy/MBq for the 1 hour and 4.8 hour bladder voiding models, respectively. The absorbed doses of 18F-HX4 to the liver, large and small intestines, and gallbladder, uterus and ovaries were in the range of 14-28 μGy/MBq in the two models with the remaining organs having lower doses. The average values of effective dose (ED) for 18F-HX4 were 14±1 μSv/MBq and 27±2 μSv/MBq respectively for the two models. For a 740 MBq injected activity of 18F-HX4, the average effective dose for the 1 hour model is 10.4±0.7 mSv and the dose to the bladder is 63±7 mSv. The dose to the bladder may be reduced by encouraging adequate hydration and frequent voiding.
The injection of 189 ± 3 MBq of 18F-HX4 in three male monkeys produced no clinically significant effects on vital signs (blood pressure, pulse, EKG) and blood tests during the 3 hour observation period following administration. In the rhesus, the liver was best visualized at approximately 4 minutes post injection, while the bladder content became visible approximately 7 minutes post injection.
PET MIP images for selected time points illustrate the nature of organ uptake as a function of time (). At 3 minutes, rapid uptake of 18F-HX4 is observed in liver, heart, kidneys, and bladder, with patchy uptake in head. Bladder activity continues to accumulate with time, as the rhesuses are under anesthesia and do not void. Rapid clearance of activities in other organs is observed in the images at subsequent time points.
Fig. 4 Decay-corrected anterior maximum-intensity projections of PET at 3, 13, 40, 77 and 187 min (from left to right) after injection of 18F-HX4 in a rhesus monkey. The liver and kidney activities decrease rapidly with time, and bladder accumulates activity (more ...)
Urinary bladder had the highest uptake, with 60 ± 2% IA at the end of the 3 hour 18F-HX4 PET/CT acquisition. Peak values of %IA in the liver and kidneys were 9.7% and 7.3% (). At the end of the study, 6.2 ± 0.1% IA was found in the gut which included the gallbladder, small intestine, proximal and distal colons' contents. The residence times for the organs are listed in . The estimated mean organ doses from 18F-HX4 are given in .
Fig. 5 Mean percentage of injected activity and standard deviation (SD) for top three organs determined on the basis of three rhesus monkey 18F-HX4 PET emission scans, as a function of time after injection. Liver and kidney activities decrease rapidly with time, (more ...)
Residence times of source organs for rhesus monkeys injected with 18F-HX4 (n=3, mean ± SD).
Radiation Doses for 18F-HX4 for the Human Adult Male (1.0 and 4.8 Hour bladder voiding Interval) [based on male primate (n=3) biodistribution data]
The four organs with the highest radiation absorbed doses of 18F-HX4 were urinary bladder wall, testes, ULI wall, and small intestine. The mean effective dose of 18F-HX4 was 25 ± 3.5 μSv/MBq and 42 ± 4.2 μSv/MBq for 1 hour and 4.8 hour bladder voiding models respectively.
18F-HX4 shows a biodistribution dominated by activity in the bladder and steady renal clearance, with 60% of IA excreted in the 3 hours of the study. In the intermediate scans rapid clearance of activity is noted in all the organs, and in the last scans intestinal elimination as indicated by gut activity can be observed. Apart from urinary bladder, liver, kidneys, heart are seen to have above background activities in the early stages, with gut activity showing in later time points. All other organs displayed near the background level of activity.
For the adult male model, only the urinary bladder wall was estimated to receive radiation doses larger than 100 μGy/MBq. The mean absorbed dose of bladder wall was 131 ± 6.1 μGy/MBq and 415 ± 18 μGy/MBq for 1.0-hour and 4.8-hour bladder voiding intervals respectively. Testes received the second highest dose at ~60 μGy/MBq. The organs in the digestive system has doses in the range of 8-32 μGy/MBq. The rest of the organs had lower doses. The mean values of the ED for the adult male were 25 ± 3.5 μSv/MBq (1.0 hour void interval) and 42 ± 4.2 μSv/MBq (4.8 hour void interval), respectively.
Comparison of Human and Monkey data
The monkey data includes fast sampling/dynamic PET scans (15 seconds per bed position) during the first few minutes followed by longer PET scans (4 minutes per bed position). The human data does not include the fast sampling but consists of sequential whole body PET scans at 6 minutes per bed position. A comparison of dosimetry for another 18
F based imaging agent using similar data sets has shown that consistent dosimetry results are obtained without the fast sampling [17
]. Hence the comparison of results between monkey and human data may be justified in spite of the difference in the acquisitions.
There are many similarities between the monkey and human data. The organs with the highest number of disintegrations per unit injected activity of 18F-HX4 are the bladder, liver and kidneys for both human and monkey data. The organ with the highest dose of 18F-HX4 is the bladder wall in both human and monkey cases. For both data the various organs of the digestive tract have similar doses. For humans, the doses of 18F-HX4 are in the range 9-24 μGy/MBq, whereas for monkeys, the doses of 18F-HX4 are in the range of 8-34 μGy/MBq. The effective dose is somewhat higher for monkeys (42 μSv/MBq) in comparison to humans (27 μSv/MBq). The estimated dose to testes is much higher in monkey data (60-65 μGy/MBq) than in human data (9-13 μGy/MBq). The monkey data is expected to overestimate the absorbed radiation dose to the testes, as monkey testes are at least twice as large as human testes.
Comparison to other 18F PET radiopharmaceuticals
shows the doses per injected activity to individual organs for 18
F-FDG (4.8 hour bladder voiding model)[18
F-FMISO and 18
F-labeled fluoroerythronitroimidazole (FETNIM) (4 hour bladder voiding model) [9
]. The absorbed doses in heart and brain are much lower for 18
F-HX4 compared to 18
F-FDG, and the absorbed dose in urinary bladder is higher for 18
F-HX4 than 18
F-FDG. A large fraction (~45% in 3.6 hours) of 18
F-HX4 is excreted through urinary system whereas 18
F-FMISO, has little urinary excretion (~4% in 5 hours). 18
F-FETNIM has an intermediate level of urinary excretion. Hence the urinary bladder dose for 18
F-HX4 (299 μGy/MBq) is much higher than that for 18
F-FETNIM and 18
F-FMISO (127 μGy/MBq and 29 μGy/MBq respectively) (for 4.8/4 hour bladder voiding models). However, several steps can be taken to reduce the bladder dose. Patients can be encouraged to void frequently as the bladder dose reduces to ~85±10 μGy/MBq in the 1 hour bladder voiding model. Having a near full bladder prior to injection can also reduce the bladder dose by diluting the bladder activity [20
Organ doses in μGy/MBq for 18F-HX4, 18F-FDG, 18F-FMISO, 18F-FETNIM.
compares whole body radiation dose and effective dose from 18
F-HX4 to dose from a few 18
F based radiopharmaceuticals [9
]. As shown in these tables, the radiation dose from 18
F-HX4 is comparable to that from other 18
F-based imaging agents, assuming similar injected activities for all the imaging agents. The effective dose of 18
F-HX4 can be reduced considerably with frequent voiding.
Table 6 Comparison of Dose between 18F-HX4 and other 18F-based imaging agents [9, 18, 19]
In summary, biodistribution of the hypoxia PET imaging agent 18F-HX4 has been measured in four humans and three monkeys using sequential PET/CT scans. Over 80% of 18F-HX4 maintains its integrity during the two hours following injection and it clears quickly through the renal system. The urinary bladder wall has the highest absorbed dose among all the organs. The dose to bladder can be reduced by encouraging adequate hydration and frequent voiding. The absorbed radiation dose from 18F-HX4 is similar to that of 18F-FDG and 18F-based hypoxia-imaging agents. Our results indicate that high quality clinical PET/CT images can be obtained shortly after the injection of 18F-HX4. 18F-HX4 biodistribution in monkeys has many similarities to its biodistribution in humans, with the estimated effective dose determined from monkey data being somewhat higher than that from human data. Considered together, its radiation absorbed dose, integrity in vivo, and rapid renal clearance make the putative hypoxia tracer 18F-HX4 favorable for further use in humans.