Abstract
We recently developed the radiotracer 4-[(3-iodophenyl)amino]-7-(2-[2-{2-(2-[2-{2-(18F-fluoroethoxy)-ethoxy}-ethoxy]-ethoxy)-ethoxy}-ethoxy]-quinazoline-6-yl-acrylamide) (18F-PEG6-IPQA) for noninvasive detection of active mutant epidermal growth factor receptor kinase-expressing non–small cell lung cancer xenografts in rodents. In this study, we determined the pharmacokinetics, biodistribution, metabolism, and radiation dosimetry of 18F-PEG6-IPQA in nonhuman primates. Methods: Six rhesus macaques were injected intravenously with 141 ± 59.2 MBq of 18F-PEG6-IPQA, and dynamic PET/CT images covering the thoracoabdominal area were acquired for 30 min, followed by whole-body static images at 60, 90, 120, and 180 min. Blood samples were obtained from each animal at several time points after radiotracer administration. Radiolabeled metabolites in blood and urine were analyzed using high-performance liquid chromatography. The 18F-PEG6-IPQA pharmacokinetic and radiation dosimetry estimates were determined using volume-of-interest analysis of PET/CT image datasets and blood and urine time–activity data. Results: 18F-PEG6-IPQA exhibited rapid redistribution and was excreted via the hepatobiliary and urinary systems. 18F-PEG6 was the major radioactive metabolite. The critical organ was the gallbladder, with an average radiation-absorbed dose of 0.394 mSv/MBq. The other key organs with high radiation doses were the kidneys (0.0830 mSv/MBq), upper large intestine wall (0.0267 mSv/MBq), small intestine (0.0816 mSv/MBq), and liver (0.0429 mSv/MBq). Lung tissue exhibited low uptake of 18F-PEG6-IPQA due to the low affinity of this radiotracer to wild-type epidermal growth factor receptor kinase. The effective dose was 0.0165 mSv/MBq. No evidence of acute cardiotoxicity or of acute or delayed systemic toxicity was observed. On the basis of our estimates, diagnostic dosages of 18F-PEG6-IPQA up to 128 MBq (3.47 mCi) per injection should be safe for administration in the initial cohort of human patients in phase I clinical PET studies. Conclusion: The whole-body and individual organ radiation dosimetry characteristics and pharmacologic safety of diagnostic dosages of 18F-PEG6-IPQA in nonhuman primates indicate that this radiotracer should be acceptable for PET/CT studies in human patients.
Over the past decade, the oncogenic role of the epidermal growth factor receptor (EGFR) has been increasingly characterized because of improved understanding of the mechanisms of receptor activation, the finding of somatic mutations of this receptor, mutations in components of the signaling pathway of the receptor, and the clinical success of anti-EGFR therapies for cancer patients (1). Therapeutic responses to anti-EGFR agents differ in patients and are sometimes contradictory, depending on the tumor type and clinical setting. Emerging data suggest that certain activating mutations in the EGFR kinase domain in patients with non–small cell lung carcinomas (NSCLC) are more frequent in nonsmokers, women, and those with adenocarcinoma histology or who are of Asian ethnicity (2–5) and are associated with responsiveness to gefitinib and other EGFR inhibitors. EGFR overexpression identified using immunohistochemistry correlates with EGFR amplification detected by fluorescence in situ hybridization but not necessarily with EGFR mutations or response to therapy with EGFR kinase inhibitors (6).
Without accurate in vivo measurements of EGFR expression and signaling activity in human tumors, it is impossible to predict the responsiveness of tumors to EGFR kinase inhibitors or to determine whether a poor tumor response to an EGFR-targeted drug results from the lack of specific activating mutations, the absence of a survival function of EGFR, insufficient long-term occupancy of the receptor by reversible inhibitors, or a high rate of receptor regeneration when using irreversible inhibitors. Although invasive tissue sampling can provide spatially limited, temporally static information about the expression or activity of EGFR at the kinase level, multiple sampling of heterogeneous tumor tissue and repeated biopsies are almost always infeasible. Consequently, interest in the use of EGFR tyrosine kinase inhibitors as radiotracers for noninvasive molecular imaging with PET of tumors that overexpress EGFR, especially tumors expressing constitutively active EGFR mutants, has been growing over the past several years (7), because noninvasive PET may facilitate repetitive quantitative assessment of the magnitude and heterogeneity of EGFR expression and activity at the kinase level in tumors in individual patients.
Several radiolabeled small-molecule agents for PET of EGFR expression at the kinase level have been reported to date (summarized in the supplemental materials, available online only at http://jnm.snmjournals.org). Recently, we reported on successful development and evaluation of a new radiotracer 4-[(3-iodophenyl)amino]-7-(2-[2-{2-(2-[2-{2-(18F-fluoroethoxy)-ethoxy}-ethoxy]-ethoxy)-ethoxy}-ethoxy]-quinazoline-6-yl-acrylamide) (18F-PEG6-IPQA) (8). PET/CT studies in mice bearing human NSCLC xenografts demonstrated that 18F-PEG6-IPQA could distinguish tumors expressing constitutively active mutant L858R EGFR that are responsive to therapy with EGFR kinase inhibitors from tumors expressing wild-type EGFR or L858R/T790M dual-mutant EGFR that are resistant to such therapy (9). Therefore, we suggest that high levels of 18F-PEG6-IPQA accumulation in NSCLC lesions should predict favorable responses to therapy with EGFR kinase inhibitors. Also, repetitive PET/CT with 18F-PEG6-IPQA could potentially be used in patients for noninvasive monitoring of pharmacodynamics of EGFR kinase inhibitors at the target level.
For the preparation of an investigational new drug application for clinical phase I imaging studies, the Food and Drug Administration regulatory guidelines require the assessment of pharmacologic and radiation safety of novel radiolabeled imaging agents. Nonhuman primates (i.e., rhesus macaques) are genetically close to humans and have similar anatomy, physiology, and metabolism. They therefore provide an ideal model for the evaluation of pharmacokinetics, metabolism, radiation dosimetry, and pharmacologic safety of novel radiotracers (10).
Here, we report the results of PET/CT studies in nonhuman primates aimed at determining the pharmacokinetics, biodistribution, metabolism, radiation dosimetry, and pharmacologic safety of diagnostic doses of 18F-PEG6-IPQA and at extrapolating to radiation dose estimates for humans.
MATERIALS AND METHODS
18F-PEG6-IPQA Preparation
18F-PEG6-IPQA was synthesized as described previously (8). 18F-PEG6-IPQA was purified using high-performance liquid chromatography (HPLC) (>97% pure). The mass of nonradioactive F-PEG6-IPQA was 0.91 ± 0.41 μg per 1 mCi (37 mBq), and the specific activity of 18F-PEG6-IPQA was 33.67 ± 12.95 GBq/μmol (0.91 ± 0.35 Ci/μmol), based on 17 radiosynthesis batches.
Experimental Animals
Six rhesus macaques (3 female and 3 male; body weight, 6–11 kg) were included in this study. All experiments were performed following institutional guidelines for conducting experiments in nonhuman primates under an Institutional Animal Care and Use Committee–approved research protocol. Before imaging, the animals were kept fasting overnight but had free access to water. To facilitate intubation, the animals were premedicated with atropine sulfate (0.04 mg/kg) intramuscularly and anesthetized with ketamine (10–15 mg/kg) intramuscularly, followed by inhalation anesthesia with isoflurane 1%–3% and oxygen delivered by an Excel 210 SE gas anesthesia system (Ohmeda Inc.). After intubation and stabilization of vital signs under anesthesia, the animals were cannulated in both saphenous veins: one cannula was used for injection of 18F-PEG6-IPQA, and the other was used for repetitive blood sampling during imaging. Also, the urinary bladder was catheterized and the catheter clamped to collect radioactive urine after the PET/CT scan. Body temperature was maintained at 37°C ± 0.8°C using an air-circulating heating device (model 505; Arizant Healthcare Inc.). The electrocardiogram, blood pressure, pulse oxymetry, and respiration rates were monitored using a Solar 8000 system (Marquette Medical Systems, Inc.) throughout the imaging study.
PET/CT Study
PET/CT studies were performed using a Discovery ST8 PET/CT system (GE Healthcare). The CT component of the study consisted of a helical scan covering the head to the mid thighs (120 kVp, 300 mA, 0.5-s rotation; table speed, 13.5 mm/rotation) with no contrast enhancement. Axial CT images were reconstructed with a slice thickness of 3.75 mm. A dynamic PET scan was then acquired starting with the onset of the 18F-PEG6-IPQA injection, which was administered intravenously. Each animal received a dosage of 141 ± 59.2 MBq (3.8 ± 1.6 mCi) of 18F-PEG6-IPQA in a volume of 5 mL of normal saline as a continuous infusion over 1 min using a digital dual-syringe infusion pump (model 33; Harvard Apparatus). Simultaneously with the 18F-PEG6-IPQA infusion, 15 mL of saline was infused using a second syringe through the same catheter port; this infusion of saline continued for another 4 min after the end of the 18F-PEG6-IPQA injection to steadily flush the infusion line (Supplemental Fig. 1). Such a dual-infusion approach was used to prevent spikes in radioactivity concentration in the blood, which usually occur because of a bolus flush after the radiotracer injection.
Dynamic PET covering the thoracoabdominal area was performed for the first 30 min, followed by 4 whole-body static images from the mid femoral position to the head at 60, 90, 120, and 180 min after the injection of 18F-PEG6-IPQA. The whole-body static PET images were acquired in 3-dimensional mode for 3 min per bed position. Also, a low-dose, unenhanced CT scan was obtained immediately after the dynamic PET scan and at the end of the last PET scan to ensure accurate coregistration of PET and CT images (Supplemental Fig. 2).
PET images were reconstructed using standard vendor-provided reconstruction algorithms that incorporated ordered-subset expectation maximization and corrected for attenuation using data from the CT component of the examination; the emission data were corrected for scatter, random events, and dead-time losses using the PET/CT scanner's standard algorithms. The dose calibrator (CRC-15R; Capintec) was cross-calibrated with the PET/CT instrument to ensure the quantitative accuracy of the PET data.
Image Analysis
Regional dynamic and whole-body reconstructed PET/CT data were stored in the Digital Imaging and Communications in Medicine 3.0, part 10, file format and transferred to a HERMES Workstation (version 2.2; Nuclear Diagnostics AB). Three-dimensional volumes of interest (VOIs) of identifiable source organs were constructed on the CT images, and their positions were verified on the corresponding PET images to include all organ activity. These VOIs were then used for PET image analysis. The identifiable source organs analyzed were the heart, liver, gallbladder, kidneys, urinary bladder, small and large intestines, brain, and whole body. Three-dimensional VOI definitions were used to visually inspect for misregistration due to motion between sequential scans in the same segment. Residual errors were manually corrected by redefining the VOIs when necessary; this was necessary only for the gallbladder and urinary bladder, which showed gradual accumulation of radioactivity as well as enlargement over the course of the PET scan.
Residence Time and Absorbed Radiation Dose Calculations
Individual organ and whole-body time–activity curves were fitted to a monoexponential or biexponential function using SigmaPlot software (version 11; Systat Software). If the data were not well approximated by exponentials, they were fitted by trapezoidal approximations of the measured data followed by exponential decay with the physical half-life from the last measured radioactivity concentration. The residence time for the exponential fits was calculated by dividing the fractional uptake by the decay constant. For biexponential fits, the residence time was the sum of this ratio for each term. For trapezoidal approximations of 18F-PEG6-IPQA, the area under the curve was calculated and divided by the administered activity of 18F-PEG6-IPQA to yield the residence time. The residence time for the blood was calculated using the time–activity curve of the drawn blood samples, assuming that 8% of the animal's mass was blood.
Because the animals were catheterized and could not void their bladders during the PET/CT scans, radioactive urine accumulated in their bladders. For each animal, we estimated the fraction in the bladder from the activity in a later PET image divided by the decayed administered activity and the biologic half-life by a fit to a single exponential increase-to-maximum fit to the decay-corrected bladder time–activity curve. These parameters were used to estimate the residence time for 18F-PEG6-IPQA in the bladder using the dynamic bladder model in the OLINDA/EXM 1.1 software program (Vanderbilt University) (11), with the assumption of a 2-h voiding interval.
The residence times, including those in the blood, were humanized using the mass of each organ estimated using the CT scans of the nonhuman primates and the mass of each organ in the MIRD 70-kg standard man model as inputs to Equation 8 according to the report by Macey (12). The blood residence time was then apportioned among the organs according to their blood volume fractions tabulated in the International Commission on Radiological Protection publication 53 (13). The human dosimetry of 18F-PEG6-IPQA was then estimated using these humanized residence times and the adult male and adult female models in the OLINDA/EXM software program.
Blood Sampling and Analyses
Immediately before 18F-PEG6-IPQA injection in each animal, a 0.5-mL blood sample was drawn to measure the hematocrit and blood gases with an i-STAT portable analyzer (Abbott Point of Care). 18F-PEG6-IPQA was injected only if the hematologic and blood chemical parameters in the blood sample were all within the reference ranges. Subsequently, 0.5-mL venous blood samples were obtained via the catheterized vein at 10, 20, 40, 60, and 90 s and 2, 3, 5, 8, 16, 32, 60, 90, 120, and 180 min after 18F-PEG6-IPQA injection. At the end of each PET/CT session, an additional blood sample (0.5 mL) was obtained and analyzed using an i-STAT portable analyzer to determine potential acute toxic side effects from the diagnostic dose of 18F-PEG6-IPQA.
The blood and plasma were assayed for radioactivity concentration using a γ-counter (Cobra Quantum; PerkinElmer). A portion of each plasma sample was extracted with acetonitrile and subjected to radio-HPLC analysis using an analytic HPLC system (model 1100; Agilent Technologies) equipped with a PET metabolite radiodetector (Flow-Count; Bioscan). The analysis was performed using a ZORBAX Eclipse XDB-C8 column (4.6 × 150 mm; Agilent Technologies) with a mobile phase consisting of acetonitrile/10 mM ammonium acetate buffer (47/53 ratio; pH 5.5) at a 1 mL/min flow rate. Under these conditions, the retention time was 7.2 min for 18F-PEG6-IPQA, whereas the major radioactive catabolite, 18F-PEG6, eluted at 1.7 min, as determined using 18F-PEG6, an authentic standard. The fraction of 18F-PEG6-IPQA versus the radiometabolite (18F-PEG6) fraction for each sample was determined on the basis of their respective radioactive peak areas. The total radioactivity in plasma was expressed as percentage injected dose per milliliter and plotted over time after injection of 18F-PEG6-IPQA. Also, the percentage injected dose per milliliter of intact 18F-PEG6-IPQA and 18F-PEG6 catabolite was calculated from the radio-HPLC results and plotted over time after injection of 18F-PEG6-IPQA to generate the corresponding time–activity curves. The percentage of 18F-PEG6-IPQA bound to red blood cells (RBCs) versus blood plasma was determined by mixing 18F-PEG6-IPQA with heparinized blood and incubating for 15 min at 37°C, separating RBC and plasma by centrifugation at 3,000 rpm for 5 min and measuring the radioactivity concentration in plasma and the RBC pellet.
Urine Analysis
Whole urine was sampled from each animal at the end of PET via a Foley catheter. The total radioactivity of each sample was measured using a γ-counter. Samples were subjected to the same radio-HPLC analysis as for the blood samples.
Assessment of Acute Toxicity of 18F-PEG6-IPQA
The vital signs, electrocardiogram results, blood pressure, pulse oxymetry, and respiration rate of experimental animals were monitored throughout the animal preparation and imaging session. Detailed physical examinations of the monkeys were performed by experienced veterinarians immediately before 18F-PEG6-IPQA PET/CT and on days 3 and 14 after 18F-PEG6-IPQA injection. Additional blood samples (5–10 mL) were drawn from each animal before and immediately after the imaging study and at 3 and 14 d after the PET study for extensive hematologic and toxicologic analyses (including liver enzymes, creatine, and nitrogen) performed by an accredited veterinary clinical laboratory.
RESULTS
The mean ages, body weights of the experimental animals, and injected 18F-PEG6-IPQA doses are listed in Table 1. All monitored clinical parameters, including heart and respiration rates, body temperature, blood pressure, electrocardiogram results, and saturation of peripheral oxygen (SPO2), were within the reference ranges throughout the experiments. During and after PET/CT and over the 14 d follow-up period, none of the animals developed any adverse events. There were no significant differences in hematologic and biochemical parameters measured before and immediately after the study or 3 and 14 d after 18F-PEG6-IPQA administration, except for a transient increase in alanine aminotransferase (ALT) on day 3, which returned to baseline level by day 14 after the PET/CT study (Supplemental Table 1).
Dynamic PET revealed the pattern and kinetics of 18F-PEG6-IPQA distribution and clearance at 180 min after intravenous administration (Fig. 1). The 18F-PEG6-IPQA–derived radioactivity peaked in the heart and kidneys within 1 min after initiation of administration and then started to accumulate in the liver and gallbladder. By 30–60 min after injection, organs involved in the hepatobiliary and renal clearance pathways dominated the whole-body distribution of 18F-PEG6-IPQA, with the highest radioactivity concentration in the gallbladder, parts of the small and upper large intestines, the kidney, and the urinary bladder. Individual source organ time–activity curves for 18F-PEG6-IPQA–derived radioactivity are shown in Figure 2. The steady-bolus injection method allowed for reliable identification of the peak of total radioactivity concentration and of the concentration in blood plasma of intact 18F-PEG6-IPQA and 18F-PEG6 catabolite (Fig. 3). 18F-PEG6-IPQA binding to RBCs was negligible; 96% ± 1% of radioactivity was recovered in blood plasma. The total blood radioactivity concentration peaked at 2.6 min after the initiation of 18F-PEG6-IPQA administration and gradually decreased thereafter, conforming to biexponential kinetics, with average half-lives of 2.0 ± 0.1 and 20.3 ± 1.6 min for the fast and slow components, respectively. The radioactivity concentration from intact 18F-PEG6-IPQA also peaked at 2.6 min after the initiation of 18F-PEG6-IPQA administration and gradually decreased thereafter, conforming to biexponential kinetics, with average half-lives of 1.6 ± 0.1 and 12.6 ± 1.2 min for the fast and slow components, respectively. The major radioactive catabolite of 18F-PEG6-IPQA in blood was identified as 18F-PEG6, the concentration of which in blood peaked at 5 min after intravenous administration of 18F-PEG6-IPQA. The clearance of 18F-PEG6 from blood followed biexponential kinetics, with half-lives of 16.1 ± 1.3 min and 74.6 ± 2.2 min for the fast and slow components, respectively. Three hours after 18F-PEG6-IPQA administration, the predominant fraction of urine radioactivity (>95%) was due to 18F-PEG6 and products derived from further breakdown of 18F-PEG6, such as 18F-fluoroacetate (3%–5%), with only minute amounts of 18F-fluoride.
To estimate the maximum dosage of 18F-PEG6-IPQA that could be safely administered to human patients, individual organ-absorbed radiation doses were calculated using the corresponding organ radioactivity residence times, τ (Table 2). The maximum possible τ was Tphysical/ln(2) = 1.443 × 109.8 = 158 min (2.64 h). The fact that the mean sum of the residence times was close to the maximum possible τ indicates that all of the administered activity was accounted for. Estimates of the human individual organ radiation-absorbed doses per unit administered activity of 18F-PEG6-IPQA are presented in Table 3.
The gallbladder received the highest absorbed dose (0.394 mSv/MBq), making it the limiting organ. The relatively high dose to the gallbladder is due to the rapid extraction of 18F-PEG6-IPQA from the circulation by the liver and thus the excretion of a large fraction of injected radioactivity into the gallbladder. The other key organs with high absorbed radiation doses were the kidneys (0.0830 mSv/MBq), upper large intestine wall (0.0267 mSv/MBq), small intestine (0.0816 mSv/MBq), and liver (0.0429 mSv/MBq). On the other hand, the skin, breast, brain, thymus, thyroid, and testes received much lower radiation doses. The effective dose was 0.0165 mSv/MBq. Therefore, administration of diagnostic dosages of 18F-PEG6-IPQA up to 128 mBq (3.47 mCi) per intravenous injection should be safe for the initial cohort of patients participating in a phase I clinical study.
DISCUSSION
In this study, the pharmacokinetics, biodistribution, metabolism, and radiation dosimetry of the new radiotracer 18F-PEG6-IPQA was assessed in healthy rhesus macaques using dynamic PET and serial blood sampling.
After intravenous administration, 18F-PEG6-IPQA did not cause acute or delayed toxicity up to 14 d, except for a transient increase of ALT 3 d after the study, which decreased back to baseline level by 14 d after the study. This transient increase in ALT is most likely due to ketamine and isoflurane anesthesia (which are known to cause transient upregulation of liver transaminases lasting for several days after anesthesia) lasting for more than 4 h (including time required for the animal preparation and transportation to and from the imaging suite) (14,15).
The lack of toxicity or any detectable pharmacologic effects from the diagnostic doses of 18F-PEG6-IPQA was expected, because the total amount of nonradiolabeled, pharmacologically active F-PEG6-IPQA administered to each animal was in the range of 3–5 μg. This mass dose is about 50,000- to 80,000-fold less than the average single therapeutic dose of gefitinib (250 mg, by mouth) and 30,000- to 50,000-fold less than that of erlotinib (150 mg, by mouth), which is administered once daily for 4 wk for therapy of patients with NSCLC (16–19).
From the dynamic PET images, it is evident that 18F-PEG6-IPQA accumulates only transiently in the organs of the hepatobiliary and renal systems, which contribute to clearance of 18F-PEG6-IPQA from the circulation. Initially, the intact 18F-PEG6-IPQA is absorbed by the liver because of its relatively high lipophilicity (LogD = 1.44). Then, the 18F-PEG6-IPQA–derived radioactivity is excreted into the gallbladder and later evacuated into the small intestine. This partially explains the reason for higher radiation doses estimated for the gallbladder, upper large intestine wall, small intestine, and liver. At this time, we do not know exactly what 18F-labeled metabolites are excreted into the gallbladder. However, after the degradation of 18F-PEG6-IPQA, the major radioactive catabolite, 18F-PEG6, is eliminated from the circulation predominantly by the kidneys into the bladder, which are also among the critical organs. Little 18F-fluoride was found in the blood and urine, as evidenced by the absence of radioactivity accumulation in the skeletal structures on PET/CT images. The background radioactivity in other organs, including the heart, was very low to negligible and decreased further with time. Thus, current radiation dosimetry estimates for 18F-PEG6-IPQA derived from nonhuman primates indicate acceptable radiation-absorbed doses to the critical organs (liver, gallbladder, intestine, kidney, and bladder) and even lower radiation-absorbed doses to radiation-sensitive organs in human patients. It should be possible to shorten the residence time and decrease the absorbed dose to the gallbladder by administering cholecystokinin or sincalide (20–22), which should allow for the safe administration of higher or repetitive dosages of 18F-PEG6-IPQA without exceeding the 50 mSv per organ per year limit for compounds that might be “generally recognized as safe” according to the Food and Drug Administration regulations (title 21 of Code of Federal Regulations part 361.1). On the basis of our estimates, diagnostic dosages of 18F-PEG6-IPQA up to 128 mBq (3.47 mCi) per injection should be safe for administration in the initial cohort of human patients in phase I clinical PET studies.
Recently, the biodistribution and radiation dosimetry of the 11C-labeled EGFR kinase inhibitor, 11C-PD153035, was assessed with PET in 12 healthy human volunteers (23). In that study, after administration of 329.3 ± 77.8 MBq of 11C-PD153035, the highest radiation-absorbed doses were observed in the urinary bladder, gallbladder, liver, small intestine, and kidney, as is consistent with our current study. However, direct comparison of results obtained in the 11C-PD153035 study with our current study is not feasible because neither blood sampling nor radioactive decay correction was performed in the 11C-PD153035 study.
18F-PEG6-IPQA accumulation was low in tissues known to express higher levels of EGFR, such as lung parenchyma, bronchi, and intestine. The lack of 18F-PEG6-IPQA accumulation in these organs can be explained by the high specificity of the tracer for irreversible binding to the active mutant L858R EGFR and its significantly lower affinity to the wild-type EGFR, as was demonstrated by us previously (9). Compared with 18F-PEG6-IPQA in rhesus macaques, other radiolabeled EGFR kinase inhibitors, such as 18F-gefitinib, demonstrated similar biodistribution in vervet monkeys (24), except for higher retention in the lungs. Similarly, 11C-PD153035 (23) exhibited higher retention in the lungs of human patients than did 18F-PEG6-IPQA. These differences are probably due to the higher affinity of 18F-gefitinib and 11C-PD153035 to wild-type EGFR kinase, as compared with 18F-PEG6-IPQA, which is more specific to mutant L858R EGFR kinase (8,9).
On the basis of the observed pattern of 18F-PEG6-IPQA biodistribution and radiation dosimetry estimates in nonhuman primates, we suggest that imaging of the L858R mutant EGFR expression in NSCLC with 18F-PEG6-IPQA should be feasible in human patients in the chest area and remote sites, except for liver metastases. PET could be initiated at 1 h after intravenous injection of 18F-PEG6-IPQA, to allow for the background activity in the liver to decline sufficiently and enable effective imaging of the lower thoracic region. Because the time–activity curves of 18F-PEG6-IPQA in the brain and muscle were similar, it is likely that this radiotracer can cross the normal blood–brain barrier by nonfacilitated diffusion and equilibrate between blood and brain tissue, which is most likely due to its fairly high lipophilicity. Consequently, 18F-PEG6-IPQA could also be evaluated for early detection of metastases of NSCLC to the brain.
Two relatively novel techniques were implemented in the current study. One technique used a dual-syringe injection pump–based steady-bolus infusion of the radiotracer, preventing the development of spikes of radioactivity concentration in the blood, which usually occurs because of a flush bolus (of saline) after radiotracer injection. The other technique used corresponding CT-based 3-dimensional VOIs to calculate the volume of organs and then verified them on the corresponding PET emission images to include all organ activity. Three-dimensional VOIs were used to visually inspect for movement artifacts between sequential scans in the same segment. Residual errors were manually corrected by redefining VOIs for the gallbladder and urinary bladder, which showed dynamic volume and radioactive concentration changes during the 180-min course of the PET study. Using these techniques, we achieved a mean total residence time that was similar to the maximum possible τ, whereas the SD of means of pharmacokinetic parameters for 18F-PEG6-IPQA and 18F-PEG6 in blood varied less than 5%, demonstrating high reproducibility.
The current study, however, has some limitations. The pharmacokinetics, biodistribution, and metabolism of 18F-PEG6-IPQA were studied in animals under general anesthesia, which could interfere with hemodynamics in certain organs and thus influence metabolism. Although blood flow to the liver is altered less by isoflurane than by other anesthesia methods, renal blood flow and urine volume are decreased by isoflurane (25). In previous PET studies of animals and humans, general anesthesia caused significant reduction in brain glucose metabolism, suggesting that conscious monkeys should be a better model for the future studies (26,27). General anesthesia could also affect the metabolism of 18F-PEG6-IPQA.
CONCLUSION
The pharmacokinetics, metabolism, biodistribution, and radiation dosimetry characteristics of 18F-PEG6-IPQA studied in nonhuman primates indicate that diagnostic dosages of this radiotracer should be safe for PET in human patients. The highest radiation dose is delivered to the gallbladder. The dose estimation and radiation doses delivered to other radiosensitive organs must be considered when evaluating the dosimetry of multiple administrations of 18F-PEG6-IPQA to human patients.
DISCLOSURE STATEMENT
The costs of publication of this article were defrayed in part by the payment of page charges. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 USC section 1734.
Acknowledgments
We thank Dana Toomey, Julie Basham, Deborah Petit, Alfredo Santiago, and Jennifer Miller for their excellent veterinary technical support and Nancy Swanston for help with PET/CT studies. We thank Karen Yoas for help in coordinating this study. This work was supported by the following grants: W81XWH-05-2-0027, 5U24CA126577, and NIH-NCI CA-016672 (Cancer Center Support grant), and the H.H. Laughery philanthropic gift.
- © 2011 by Society of Nuclear Medicine
REFERENCES
- Received for publication December 16, 2010.
- Accepted for publication January 31, 2011.