First-in-human healthy volunteer dosimetry studies of the excitatory amino acid transporter 2 (EAAT2) PET imaging tracer methyl N⁴-(7-[¹⁸F] fluoro-9H-fluoren-2-yl)asparaginate, [¹⁸F]RP-115

Abstract

Objective and background:

The objective of this first-in-human study was to investigate the radiosynthesis, and the preliminary safety, biodistribution, and organ radiation dosimetry of the positron emission tomography (PET) imaging tracer methyl N⁴-([¹⁸F]7-fluoro-9H-fluoren-2-yl)asparaginate, known as [¹⁸F]RP-115, in a small cohort (n=8) of healthy volunteers. The [¹⁸F]RP-115 tracer is a methyl ester prodrug and undergoes metabolic saponification in the central nervous system to generate the corresponding carboxylic acid form that selectively binds to the excitatory amino acid transporter 2 (EAAT2) protein.

Procedures and methods:

A multi-step high molar activity tracer radiosynthesis was devised to produce doses. Eight healthy human participants (four male and four female), aged 56–75, received a bolus intravenous injection of [¹⁸F]RP-115 (administered activity range: 70.3–355 MBq) prior to a total of 94 min of PET-MR scanning performed as three sequential scanning sessions. Regional tissue volumes of interest were defined, time-integrated activity coefficients (TIAC) were derived, and then estimates of organ and tissue activities and effective doses (whole body) were calculated, with two versions of OLINDA software (1.1 and 2.0) that incorporated two tissue weighting factor sets (ICRP-60 and −103), respectively.

Main findings:

Tracer was routinely produced in good radiochemical yields and as suitable high molar activity doses for injection. The [¹⁸F]RP-115 injections and PET-MR scans were well-tolerated and no adverse events were reported (≤48 h). Radioactivity was widely biodistributed with good organ uptake. TIACs and estimated radiation organ doses were determined, for which a few statistically significant estimated organ dose differences between males and females were noted. The kidneys were identified as the critical target organ.

Principal conclusions:

Injection of [¹⁸F]RP-115 was considered safe. The estimated kidney radiation doses relative to administered radioactivity identified a more optimal human [¹⁸F]RP-115 tracer injected amount of <211 MBq. This more optimal [¹⁸F]RP-115 tracer injected activity definition is similar to the amounts used for other established [¹⁸F]labeled clinical PET tracers such as [¹⁸F]FDG, and it will be used in future RP-115 clinical PET imaging studies.

Graphical Abstract

1. Introduction

The amino acid L-glutamate (L-Glu) is the major neurotransmitter in the central nervous system (CNS) and is responsible for excitatory synaptic-astrocyte tripartite cellular events [1–5]. The excitatory amino acid transporter 2 (EAAT2, GLT-1) is expressed in the CNS primarily on astrocytic cell membranes (~90 %, the balance on neurons), clears >80 % of synaptic L-Glu [1,4–6], and plays other important roles in the CNS [2,8–10]. Reduction of EAAT2 availability (e.g., trafficked off cell membrane, reduced transcription, decreased translation, and loss of cell type) is thought to dysregulate L-Glu homeostatic CNS concentrations, resulting in deleterious tripartite synaptic events [2,5,11–19], including excitotoxic processes that promote neuron death and astrocyte cell activation and/or loss [1–3,6–7,18]. When healthy human post-mortem frontal cortical tissues were studied in-vitro using the EAAT2-selective substrate [³H]D-aspartic acid [4], measured EAAT2 B_max values were ~ 30 pmol/mg protein [20]. This transporter density is considered suitable for quantification using positron emission tomography (PET) imaging with a tracer possessing low nanomolar target affinity [21–22]. Since EAAT2 is a promising CNS target to interrogate astrocytic changes in neurodegenerative disease, our focus has been to discover and develop a first-in-class fluorine-18 EAAT2 astrocytic PET imaging tracer suitable for clinical research applications. A tracer with these characteristics would enhance the field of astrocyte PET imaging [11–13].

We recently described [23–26] a new and low molar activity fluorine-18 CNS EAAT2 PET imaging tracer known as [¹⁸F]RP-115, as shown in Fig. 1 structured as a prodrug compound [¹⁸F]1; that is, composed with a terminal carboxylic methyl ester moiety. The methyl ester group allows [¹⁸F]RP-115 CNS tissue penetration. Subsequent methyl ester metabolic hydrolysis by putative CNS esterases forms the cognate carboxylic acid, compound [¹⁸F]2 that binds to EAAT2 target [25]. In-vitro pharmacological competitive EAAT2 transport inhibition assays with the non-radioactive form of the carboxylic acid against [³H] D-aspartic acid (EAAT2 expressed in COS 17.2 cells) reveal an IC₅₀ = 3 nM [25], and its EAAT2 target selectivity is considered high as determined competitively against 70 other CNS binding sites (no binding at 1 μM) [25].

Fig. 1.

Tracer [¹⁸F]RP-115, methyl N⁴-(7-[¹⁸F]fluoro-9H-fluoren-2-yl)asparaginate, compound [¹⁸F]1 and the CNS carboxylic acid metabolite, compound [¹⁸F]2.

Preclinical PET imaging animal assessments with a low molar activity form of [¹⁸F]RP-115 (e.g., ~ 0.0888 MBq/μmol) confirmed this prodrug is an effective CNS tracer for targeting EAAT2 [23–25]. For example [25], intravenous (i.v.) injection of [¹⁸F]RP-115 in wild-type Sprague-Dawley rats or rhesus monkeys followed by PET-CT imaging revealed high standardized uptake value (SUV) [27] CNS activity uptake (SUV >1), good regional cerebral and spinal tissue activity distributions as well as tissue activity washout over the course of ~45–90 min after injection. The pre-treatment challenge using the non-radioactive compound RP-115 in rats (e.g., 7 mg/kg 10 min prior to tracer), demonstrated the in-vivo EAAT2 target specificity in CNS tissues [25]. Additionally, preliminary rat arterial blood evaluations for parent tracer [¹⁸F]RP-115 resulted in the definition of an input function that has been used to calculate brain distribution volume (V_T) estimated values, in which the estimates serve as an initial basis for EAAT2 quantitation by established parametric kinetic and non-parametric graphical comparative methods [28].

Further, a GLP 14-day toxicological study with the non-radioactive compound RP-115 given on two consecutive days in Sprague-Dawley rats produced no overt adverse effects and the no-observed adverse effect level (NOAEL) was determined to be 0.111 mg/kg/day (0.666 mg/m²/day). Based on all of these preclinical determinations, [¹⁸F]RP-115 was considered a suitable PET tracer for further development by the significant improvement of the [¹⁸F]RP-115 radiosynthesis by automated means to produce a higher molar activity form and utilizing established animal-to-human tracer safety translation methods [29–33]. In this paper, we describe the automated radiosynthesis of tracer [¹⁸F] RP-115 with high molar activity and its first-in-human use in a small cohort of healthy volunteers to evaluate safety and organ dosimetry for PET imaging. This includes monitoring for any adverse events in healthy volunteers after injection, analyzing activity biodistribution profiles and determining key estimated organ dosimetry data as a function of the injected tracer amounts.

2. Materials and methods

2.1. [¹⁸F]RP-115 tracer radiosynthesis and dose preparation

Tracer [¹⁸F]RP-115 was prepared at the University of California San Francisco (UCSF) Radiopharmaceutical Production Facility according to the approved Food and Drug Administration (FDA) exploratory investigational new drug (eIND, number 156099) Chemistry Manufacturing and Control (CMC) procedures. The entire automated [¹⁸F]RP-115 synthesis, permitting the production of a high molar activity form of tracer, was carried out as shown in Fig. 2 with a Sofie Biosciences ELIXYS FLEX/CHEM system. The cassettes and solid phase extraction (SPE) cartridges were preloaded into the system and the synthesis was remotely controlled. In general, all solvents, most of the reagents and some components were acquired from Sigma-Aldrich (St. Louis, MO, USA), whereas other reagents and select components were obtained as described herein.

Fig. 2.

The multi-step radiosynthetic route used for the preparation of tracer [¹⁸F]RP-115.

Briefly, [¹⁸F]fluoride ion was produced in the UCSF GE PETrace Cyclotron (50 μA for 20–40 min) bythe ¹⁸O(p,n)¹⁸F reaction on enriched ${\mathrm{H}}_2^{18}\mathrm{O}$ and transferred to the Sofie Biosciences ELIXYS FLEX/CHEM module. The [¹⁸F]fluoride ion in oxygen-18 water was trapped on a QMA light cartridge (K920; ABX Advanced Biochemical Compounds, Radeberg, Germany) and released with 1.0 mL of acetonitrile:water (1:1) containing kryptofix 222 (K222, 6.2 mg) and potassium carbonate (1 mg). The solvent was removed by heating at 105 °C under a stream of nitrogen and reduced pressure. Additional acetonitrile (1.0 mL) was added once more and the solvent was evaporated to dryness. The residue was cooled to room temperature and a solution of 2, 7-dinitro-9-fluorenone (5–7 mg) in dimethylsulfoxide (500 μL) was added and heated at 180 °C for 10 min. Then, in a stepwise fashion, hydrazine hydrate (400 μL) and KOH in MeOH (50 mM, 400 μL) were added to the reaction vial and the mixture was heated at 180 °C for 10 min. The reaction mixture was cooled, diluted with 4 mL of HPLC grade water, and loaded onto a preconditioned (ethanol:water, 5 mL:10 mL) C18 plus short Sep-Pak (Waters Associates; Milford MA, USA).

The Sep-Pak was washed with 3 mL of water and eluted with 2 mL of acetonitrile into a second reaction vessel. The acetonitrile/water was azeotropically removed at 105 °C under a stream of nitrogen and reduced pressure. Additional acetonitrile (1.0 mL) was added and the solution was evaporated to dryness. To the dried reaction mixture vial was added a solution of N-tert-butoxycarbonylaspartic acid α-methyl ester (10–12 mg; Chem-Impex, Wood Dale, IL, USA), N,N-diisopropylethylamine (DIPEA, 7 μL), and O-(7-Aza-1H- benzotriazole-1-yl)- N, N, N′, N′-tetramethyl-uraniumhexafluorophosphonate (HATU,14–16 mg; Alfa-Aesar, Haverhill, MA, USA) in 500 μL dry acetonitrile. This mixture was stirred at 30 °C for 10 min. Subsequently, trifluoroacetic acid (650 μL) was added and the mixture was stirred at 30 °C for 10 min. The reaction mixture was diluted with 3 mL of water and injected onto a Phenomenex Luna C18(2) (250 × 10 mm, 5 μ; Phenomenex, Torrence, CA, USA) HPLC column at a flow rate of 6 mL/min (50:50:0.1 % methanol/water/trifluoracetic acid), detector monitoring UV at a wavelength of 254 nm. The RP-115 product was collected from the HPLC, diluted with sterile water for injection (SWFI; 20 mL), and passed through a C18 light Sep-Pak (Waters Associates).

For dose preparations, this C18 cartridge was washed with 5 mL of SWFI and then eluted with 1 mL of ethanol (USP). The ethanol portion containing RP-115 was eluted into a vial containing 0.5 mL of 1 mg/mL ascorbic acid in sterile water for injection and 6.5 mL of 0.9 % saline (Hospira; Pfizer, New York, NY, USA). The final solution was diluted with 5 mL of 0.9 % saline (Hospira) to give a final solution of <10 % ethanol in saline and ascorbic acid. The decay-corrected radiochemical yields were determined. Thereafter, the final product solution was passed through a Millex GV vented sterile filter (0.22 μm; MilliporeSigma) into the sterile final product vial. An aliquot was removed from the final product vial and quality testing was performed, whereby each batch of [¹⁸F]RP-115 was tested prior to release for human use.

The quality test acceptance data included: appearance (clear, colorless and no particulate matter), radiochemical identity (HPLC, relative standard retention time = 1.00 ± 0.05), radiochemical purity (HPLC ≥90 %), radionuclidic identity (half-life, 106.5–113.1), radionuclidic purity (gamma spectroscopy, NLT 99.5 % of gammas correspond to 511 keV, 1022 keV, and Compton scatter), pH (4.5–7.5), endotoxin content (PTS Endosafe, USP <85>; < 14 EU/mL), residual solvents (gas chromatography), residual reagents: Kryptofix 222 (staining, ≤ 50 μg/mL) and hydrazine (staining, ≤ 0.3 μg/mL), sterility (no signs of bacterial growth out to 14 days, aerobic and anerobic media) and filter integrity (bubble point ≥50 psi).

The final [¹⁸F]RP-115 drug product used for intravenous (i.v.) injection doses consisted of 44–463 MBq RP-115/mL in a solution consisting of USP ethanol (1 mL)/0.5 mg of ascorbic acid (0.5 mL SWFI) in 0.9 % sodium chloride for injection (11.5 mL), sterile filtered and contained in a 20 mL USP Type 1 glass, sterile and pyrogen free vial, sealed with rubber septa and crimped with an aluminum crimp. The designated expiration time for the [¹⁸F]RP-115 final drug product was 4 h after end-of-synthesis (EOS), when stored at ambient temperature in a shielded container as defined by HPLC stability determinations.

2.2. Volunteer healthy control participants

All human participant studies were conducted in accordance The Code of Ethics of the World Medical Association (Declaration of Helsinki) that were in compliance with relevant laws and institutional guidelines, including the UCSF Institutional Review Board (IRB, # 21–33973) and radiation safety committee approved protocols. The clinical research safety and dosimetry studies were conducted under the approved FDA IND 156099 (clinical trial registration number NCT05374278; ClinicalTrials.gov). Informed consent was obtained from all volunteer participants. Healthy volunteer participants were enrolled, satisfying the following inclusion and exclusion criteria that were established for this initial safety study, and also for our ongoing and anticipated future investigations with [¹⁸F]RP-115.

Inclusion criteria:

a) age 40–75, b) body mass index (BMI) that allowed for comfortable and safe positioning in the PET/MR scanner, c) the ability to provide written informed consent or have a legally authorized representative/guardian who provides surrogate informed consent, d) no apparent physical disorder, (e) peripheral vein suitable for catheterization; and f) devoid of central nervous system-affecting prescription drugs for three weeks.

Exclusion criteria:

a) inability to provide written informed consent, or not having a legally authorized representative /guardian who can provide surrogate informed consent; b) inadequate venous access; c) receipt of radioisotope <5 half-lives within RP-115 imaging- as to not confound any scans with radiation background from previous scanning, and unsuitable organ dosimetry thresholds from previous (>two weeks) PET scans; d) the performed [¹⁸F]RP-115 scan(s) must not represent >3 PET studies total within one year; e) contra-indication to magnetic resonance, including permanent pacemaker, implantable metallic device, etc.; or severe claustrophobia; f) pregnancy (female patients of childbearing age will be tested prior to injection of tracer- positive test excludes from the study); g) breast-feeding h) any medical condition or other circumstances that in the opinion of the project physicians would significantly decrease chances of obtaining reliable data, achieving the study objective or completing the study.

2.3. Participant clinical evaluations, tracer administration and image acquisitions

Participants’ baseline ECG and vital signs, height, weight, and related demographics were recorded prior to i.v. injection of the [¹⁸F] RP-115 tracer dose. A venous catheter was placed by inserting a needle into the subject’s peripheral arm vein (cephalic) and then the participant was placed supine on the scanner bed (craniocaudal, head first). A 70.3–355 MBq of [¹⁸F]RP-115 tracer was given by i.v. injection (bolus, 1–2 min). Immediately thereafter, whole-body (WB, cerebrum to mid-thighs) PET- MR concurrent scanning was performed in a GE SIGNA PET/MR system (PET: 60 cm transaxial and a 25-cm axial field of view, sensitivity >20 cps/kBq; MR: Discovery MR750w.3.0 T, four coil large field of view, 50 × 50 × 50 cm).

As this was an initial small cohort safety and organ dosimetry study, blood and urine were not sampled. A total of 94 min of WB PET-MR scanning employed contiguous bed positions with a protocol of three sequential temporal scanning sessions that concluded at 214 min after tracer injection. The PET-MR scanning sessions were as follows. Session 1: the first seven WB scans (WB1 - WB7) were performed consecutively after tracer injection as: WB1 (0–6 min, 1 min per bed position × 6 bed positions), WB2 (7–13 min, 1 min per bed position×6 bed positions), WB3 (14–20 min, 1 min per bed position × 6 bed positions), WB4 (21–27 min, 1 min per bed position × 6 bed positions), WB5 (28–34 min, 1 min per bed position × 6 bed positions), WB6 (35–47 min, 2 min per bed position × 6 bed positions) and WB7 (48–60 min, 2 min per bed position × 6 bed positions). The study participant offered a ~15 min break, during which they were given the opportunity to void (if so, noted) and drink water. Subsequently, for Session 2, WB8 scan (80–104 min, 4 min per bed position × 6 bed positions) was performed, followed by offering a second break of ~70 min (water, food and voiding permitted; if so, noted). Finally, for Session 3, WB9 scan (180–214 min; 4 min per bed position × 6 bed positions) was performed.

Study participants were observed continuously for any signs of adverse events (AE) during the course of the scanning, breaks and after completion of imaging. Vital signs and ECG were taken ~60 min after completion of the imaging session. A physician or nurse evaluated the participant prior to discharge to assess any immediate AE, followed by a participant off-site follow-up AE assessment consultation within 24–48 h after tracer injection. Additionally, participants were requested to report any AE occurring beyond the 48 h period (e.g., days to months afterward) to the study coordinator.

2.4. Data processing, and radiation biodistribution with dosimetry determinations

The GE SIGNA MR-based attenuation correction (MRAC) employed three methods, including:1) tissue classification using Dixon MRI; 2) ZTE (zero echo time) for skull and air interface delineation in the head portion of the data [34]; and 3) respiratory phase-matched Dixon MRI for the thorax portion [35]. The attenuated corrected reconstructed images were transferred from the PET-MR to the UCSF institutional picture archiving and communicating system (PACS) in DICOM format. The DICOM files were retrieved via the Automated Image Retrieval (AIR) web portal that has direct access to the institutional PACS. An appropriate HIPAA-compliant de-identification profile that allows scan time, radiopharmaceutical administration information and gender was applied during this data retrieval. Additionally, all DICOM image data were decompressed during the download to be compatible with other DICOM viewers used for further data processing. The DICOM files were sorted with Horos software (Version 3.3.1) [36] and then loaded into ITK-SNAP software (Version 3.8.0) [37] and AMIDE software (Version 1.0.5) [38]. Segmentation was performed using AMIDE in combination with ITK- SNAP, guided by general human anatomical MR determined landmarks, to ensure accuracy and reliability in the quantitative analyses.

The sorted DICOM files and segmentation images (as templates) were processed with AMIDE to determine the organ volumes of interest (VOIs) and to initially assess radioactivity biodistribution reported as SUV at early times (<1 h). The VOIs included: brain, gallbladder wall, small intestine, heart contents, heart wall, kidneys, liver, lungs, pancreas, spleen, thymus, thyroid, urinary bladder contents, uterus/uterine wall, other organs as needed, the remainder and whole body. Decay-corrected percent injected activity (%IA) per organ VOIs over time (h) was calculated with AMIDE. The %IA of the remainder was obtained by subtracting the sum of the %IA of measured organs from the whole body.

Time-integrated activity coefficients (TIACs) were computed with the exponential model (EXM) in OLINDA software (Version 1.1) [39]. Radiation doses per organ were estimated with the TIACs, by means of the 70 kg adult male or 55 kg female models in OLINDA Version 1.1 and ICRP-60 tissue weighting factors, and also the 73 kg adult male or 60 kg female models in OLINDA Version 2.0 and ICRP-103 tissue weighting factors [38,40]). Effective doses (i.e., whole body) recorded as mSv were computed from organ absorbed doses multiplied by the respective tissue weighting factors. R Statistical Software (Version 4.1.1) [41] was used to generate plots and perform statistical analyses.

3. Results

3.1. Tracer RP-115 preparation

The automated [¹⁸F]RP-115 tracer production was routine (n = 8) and characterized by the following determinations (mean ± SD): radioactivity produced 3738 ± 2223 MBq (range 566–7474); end of synthesis (EOS) yield 13.7 ± 7.1 % (range 3.70–24.4) decay corrected to starting [¹⁸F]fluoride ion at the end of bombardment; EOS molar activity 112 ± 81GBq/μmol (range 36.6–262); radiochemical purity >98 % ± 2 (range 93–99) and EOS synthesis time 163 ± 15 min (range 145–186).

3.2. Healthy control participant profiles, [¹⁸F]RP-115 injected activity data, clinical observations and biodistribution profile

Four male and four female healthy volunteers aged 56–75 were enrolled in the study as described in Table 1, where respective injected tracer activity data are provided. The study cohort age range was a result of the volunteer open recruitment process that complied with the general inclusion/exclusion criteria. All study participants voided during the second break of the scanning protocol (between WB8-WB9 scans) except female 2 (F2). There were no adverse events (AE) or clinically detectable pharmacological effects reported for any of the participants during or after the tracer dose administration (≤48 h) nor for any times beyond the 48 h period (e.g., up to 1 year after scanning). Each participant completed the scan session without reporting discomfort that would warrant cessation of scanning. Radioactivity biodistribution, reported as standardized uptake values (SUV) after injection of [¹⁸F]RP-115, was initially assessed typically 27–34 min after injection, where two examples (one male and one female) are shown in Fig. 3.

Table 1.

Data of the healthy control (HC) male (M) and female (F) participants, the respective RP-115 injected activity, mass dose, and related statistics (mean ± SD) as shown; all participants voided during the second break of the scanning protocol (between scans WB8 and WB9) except Female 2 (indicated as F2^#).

HC sex & participant number	M1	M2	M3	M4	F1	F2#	F3	F4	Mean ± SD
HC age (yr)	70	75	73	62	67	57	56	68	66 ± 7
HC body weight (kg)	78.2	92.9	68.0	107	70.5	59.0	81.6	61.7	77.3 ± 16.2
Dose radioactivity (MBq)	326	289	70.3	153	189	355	117	167	208 ± 103
Dose (mL)	4.00	9.00	0.415	0.797	1.70	1.50	0.500	0.330	2.28 ± 2.97
Concentration (MBq/mL)	83.6	44.4	181	217	141	287	296	463	214 ± 134
Molar activity (GBq/μmol)	46.6	36.6	174	167	48.7	65.0	97.5	262	112 ± 81
Tracer mass (μg)	2.53	4.39	0.140	0.330	2.10	1.74	0.540	0.220	1.50 ± 1.49
Mass dose (ng/kg)	32.3	47.2	2.09	3.11	29.7	29.5	6.62	3.58	19.3 ± 17.4

Fig. 3.

Example coronal and sagittal view PET images (near respective midlines, MR data excluded for clarity) of a male (Panel A) and female (Panel B) healthy control participant showing radioactivity biodistribution after injection (per Table 1) determined as standardized uptake values (SUV) with defined SUV gradient color bar; Panel A: male 4 (M4), 28–34 min after a 153 MBq tracer injection; Panel B: female 4 (F4), 27–33 min after a 167 MBq tracer injection.

3.3. Organ time-integrated activity coefficients

The time-integrated activity coefficients (TIACs) were computed using the curve-fitting exponential model using the EXM (Exponential Modeling) module in the OLINDA/EXM package (Version 1.1; see: Appendix, Fig. A1 for the decay corrected plots used for TIAC determinations). Per Table 2, the determined TIACs were generally comparable. Therefore, the OLINDA/EXM derived TIACs were utilized to derive the estimated dose per organ.

Table 2.

Representative select organ time-integrated activity coefficients (TIACs) determined as MBq-h/MBq, derived from the curve-fitting exponential models with OLINDA/ EXM for the healthy control (HC) male (M) and female (F) participants, in which all participants voided during the second break of the scanning protocol (between scans WB8 and WB9) except Female 2 (indicated as F2^#).

HC sex & participant number	M1	M2	M3	M4	F1	F2#	F3	F4
Organ
Brain	0.0947	0.0531	0.0611	0.078	0.084	0.075	0.0606	0.0781
Gallbladder wall	0.00525	0.0017	0.00199	0.00356	0.0031	0.00204	0.00328	0.00062
Small intestine	0.161	0.00196	0.0651	0.12	0.0843	0.0474	0.0667	0.126
Heart contents	0.00354	0.0248	0.0027	0.0033	0.00463	0.00054	0.00112	0.0015
Heart wall	0.0301	0.202	0.0297	0.0276	0.0155	0.0118	0.0131	0.0212
Kidneys	0.351	0.424	0.286	0.332	0.25	0.391	0.266	0.245
Liver	0.34	0.19	0.249	0.229	0.209	0.202	0.262	0.192
Lungs	0.322	0.0012	0.298	0.214	0.274	0.31	0.186	0.231
Pancreas	0.0022	0.111	0.00353	0.00723	0.00899	0.00377	0.00504	0.0053
Spleen	0.0528	0.0158	0.0218	0.0354	0.0284	0.00188	0.011	0.0057
Thymus	0.00576	0.0013	0.00217	0.00492	0.00372	0.00178	0.00146	0.0021
Thyroid	0.0007	0.00039	0.00057	0.00105	0.00052	0.0047	0.00131	0.0009
Urinary bladder contents	0.00668	0.017	0.0429	0.0178	0.138	0.127	0.00659	0.0319
Uterus/Uterine wall	–	–	–	–	0.00432	0.0564	0.0115	0.0133
Remainder	1.27	1.37	1.29	1.39	1.28	9.93	1.44	1.22

3.4. Estimates of organ doses

As shown in Table 3, the estimated organ absorbed doses and the remainder (mGy) and also the effective dose (whole body, mSv), were computed from the Table 2 EXM TIAC values and OLINDA software versions 1.1 (ICRP-60 weighting factors) and 2.0 (ICRP-103 tissue weighting factors). The Table 3 data from both OLINDA software versions are shown side-by-side to allow for facile numerical comparisons. The absorbed dose data (Table 3) was computed into sex composite values mean ± SD (n = 4 for each sex) as mGy/MBq, with effective dose (whole body) values computed as mSv/MBq (see numerical values shown in the Appendix, Table A1). The composite sex data sets were plotted as descending values (Fig. 4, Panel A: OLINDA 1.1, ICRP-60 weighting factors; Panel B: OLINDA 2.0, ICRP-103 weighting factors) in which the p-values <0.05 are indicated; numerical data is shown in Appendix Table A1.

Table 3.

Estimated organ absorbed doses (mGy) for the healthy control (HC) male (M) and female (F) participants, as a function of respective radioactivity given per participant (also per Table 1, above) and the OLINDA software (versions 1.1 and 2.0) used. Effective doses (i.e., whole body values) are shown in mSv. All participants voided during the second break of the scanning protocol except Female 2 (F2^#). Notable elevated numerical values found relative to the other same sex participant values are shown boldfaced, and no OLINDA software data outputs are shown as two dashes (–).

HC sex & # (activity given, MBq)	M1 (326)		M2 (289)		M3 (70.3)		M4 (153)		F1 (189)		F2# (355)		F3 (117)fs		F4 (167)
OLINDA software version	1.1	2.0	1.1	2.0	1.1	2.0	1.1	2.0	1.1	2.0	1.1	2.0	1.1	2.0	1.1	2.0
Target organs (mGy)
Adrenals	6.51	13.5	4.94	12.3	1.21	2.34	3.04	5.67	3.89	5.94	7.74	11.7	2.50	3.65	3.19	4.53
Brain	5.63	5.47	3.03	2.94	0.820	0.810	2.44	2.17	3.40	3.21	5.65	5.33	1.60	1.52	2.79	2.64
Breasts	2.73	–	2.26	–	0.560	–	1.31	–	1.79	1.71	2.93	2.78	1.13	1.09	1.47	1.41
Esophagus	–	5.18	–	4.39	–	1.00	–	2.06	–	2.98	–	5.40	–	1.78	–	2.41
Eyes	–	2.26	–	1.91	–	0.450	–	1.08	–	1.59	–	2.43	–	1.01	–	1.33
Gallbladder wall	9.08	10.3	6.41	6.09	1.35	1.53	3.80	3.76	4.40	5.09	7.35	9.31	2.95	3.38	3.08	3.61
Colon, left	–	5.63	–	5.05	–	0.970	–	2.46	–	2.85	–	4.83	–	1.77	–	2.41
Colon, right	–	4.95	–	4.10	–	0.900	–	2.15	–	2.77	–	4.76	–	1.81	–	2.41
Heart wall	8.79	9.25	6.46	33.5	1.79	1.84	1.79	3.73	4.25	4.40	6.61	6.93	2.26	2.27	4.15	4.15
Kidneys	72.0	70.7	37.8	74.2	12.7	12.4	5.75	31.2	32.5	32.6	93.4	93.4	21.0	21.4	28.1	28.1
Liver	16.0	17.5	16.6	10.1	2.61	2.85	2.12	5.85	7.85	8.25	14.6	15.5	5.81	6.07	6.41	6.74
Lower large intestine wall	3.45	–	2.97	–	0.670	–	2.51	–	2.64	–	4.26	–	1.47	–	2.09	–
Lungs	19.7	17.0	11.0	2.65	3.91	3.35	4.01	5.53	12.3	10.2	25.5	21.1	5.47	4.63	9.27	7.74
Muscle	3.00	–	2.55	–	0.605	–	35.0	–	2.06	–	3.45	–	1.29	–	1.67	–
Osteogenic cells	4.07	2.80	3.58	2.33	0.837	0.557	5.85	1.28	2.94	1.77	4.69	2.97	1.94	1.11	2.46	1.45
Ovaries	–	–	–	–	–	–	–	–	2.83	2.77	5.22	5.90	1.63	1.54	2.39	2.17
Pancreas	6.41	5.93	4.30	44.4	1.33	1.15	1.52	3.39	6.10	5.49	7.78	7.60	2.85	2.79	3.95	3.86
Prostate	–	3.21	–	2.91	–	0.724	–	1.60	–	–	–	–	–	–	–	–
Rectum	–	3.35	–	2.77	–	0.702	–	1.64	–	3.34	–	7.07	–	1.46	–	2.14
Red marrow	3.42	3.48	2.76	2.76	0.662	0.692	2.02	1.54	2.15	2.26	3.77	4.01	1.36	1.39	1.79	1.84
Salivary glands	–	2.64	–	2.20	–	0.536	–	1.26	–	1.69	–	2.62	–	1.09	–	1.41
Skin	2.10	–	1.85	–	0.434	–	4.45	–	1.44	–	2.31	–	0.945	–	1.19	–
Small intestine	13.8	17.2	9.32	4.04	1.61	1.93	1.69	6.45	6.06	7.43	8.35	9.87	3.30	3.98	6.82	8.53
Spleen	21.3	24.9	7.24	9.84	2.28	2.65	2.12	8.39	8.87	10.1	5.86	7.49	3.02	3.58	3.11	3.75
Stomach wall	4.33	5.27	3.41	5.89	0.815	0.984	1.10	2.22	2.68	3.08	4.40	5.08	1.68	1.87	2.19	2.46
Testes	2.12	2.11	2.02	2.02	0.479	0.477	7.89	1.09	–	–	–	–	–	–	–	–
Thymus	16.7	14.7	4.68	5.86	1.64	1.58	1.19	5.87	7.08	7.59	7.57	8.99	2.23	2.50	4.01	4.47
Thyroid	3.39	4.36	2.23	2.40	0.643	0.830	7.36	2.28	1.88	2.36	17.0	17.5	2.03	2.20	2.31	2.53
Upper, lower intestine wall	5.37	–	4.24	–	0.879	–	2.17	–	3.09	–	5.04	–	1.91	–	2.83	–
Urinary bladder wall	3.65	3.68	4.70	4.70	1.96	2.00	2.81	2.61	18.8	16.2	32.9	28.7	1.57	1.54	4.93	4.45
Uterus	–	–	–	–	–	–	–	–	4.21	4.55	47.2	47.6	3.93	3.86	6.49	6.39
Remainder	4.10	–	3.32	–	0.780	–	1.96	–	2.62	–	4.65	–	1.61	–	2.14	–
Effective dose (mSv)	6.74	6.65	5.48	4.65	1.52	1.25	2.89	2.62	5.42	4.54	10.6	9.27	2.67	2.27	3.90	3.28

Fig. 4.

Descending bar graph plots of computed estimated absorbed dose values (mGy/MBq) reported as mean ± SD (n = 4, per sex), shown as male (blue) and female (red) for the Table 3 organs and remainder. Effective dose (whole body) computed as mSv/MBq. The select male vs. female P-values found <0.05 (shown as *) indicate a significant distinction between the sexes. Panel A: OLINDA 1.1, ICRP-60 weighting factors. Panel B: OLINDA 2.0, ICRP-103 weighting factors. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Discussion

4.1. Tracer [¹⁸F]RP-115 preparation, injected activities, clinical observables and activity biodistribution

The automated radiosynthesis readily afforded i.v. doses of [¹⁸F]RP-115 with suitable molar activity. Initial volunteer recruitment of four male and four female healthy control participants met the established enrollment criteria. These criteria were defined for our [¹⁸F]RP-115 comprehensive PET investigations that are projected to be composed of larger and more diverse cohorts of older healthy control adults and neurodegenerative disease (e.g., Alzheimer’s, and related) patients. Thus, an evaluation of the full age range criteria (e.g., 40–75 years) composed of suitable participant diversity is planned. In the current study, the small participation sample size was considered a limitation for some of the statistical analyses. Various doses of radioactivity, concentrations and masses were administered (Table 1) to assess potential adverse events and/or pharmacological effects, radioactivity biodistribution profiles and estimated organ radiation dose values.

During and after [¹⁸F]RP-115 injection (e.g., ≤48 h, and also for up to 1 year), there were no adverse events or clinically detectable pharmacological effects observed or reported for any of the participants. Each participant completed the scan session without reporting any discomfort that would warrant scanning cessation. Together these observations revealed that the doses of [¹⁸F]RP-115 given and the PET-MR scanning protocol were well tolerated. All of the participants, except Female 2 (F2), voided during the second imaging break of the scan session. Thus, particular attention to estimated radiation dose values of urinary excretion organs was made, vide infra. Evaluation of SUV radioactivity distributions in both male and female participants at nearly equivalent times after tracer injection (e.g., 27–34 min), per the Fig. 3 example, revealed good (SUV > 1) radioactivity uptake and distribution into the major organs, the CNS and peripheral tissues. Of particular interest was the radioactivity found in the brain and spinal tissues with SUV > 1, which we consider a suitable CNS activity signal.

4.2. Organ time-integrated activity coefficients

Organ activity data plots, composed of decay-corrected percent injected [¹⁸F]RP-115 activity vs. time (h), as shown in Fig. A1 were generated. These plots revealed that the respective organ activity curves across the participants were comparable. Good brain uptake was found where activity levels remained elevated. Based on other ongoing studies, this is thought to be a function of the tracer dose amount and molar activities used in relation to the participant weight; i.e., ng of tracer mass injected vs. kg participant mass. A few outlier curves (see below) were observed for specific participants and select organs, including the: kidney (1), gallbladder (1), thyroid (1), pancreas (2), urinary bladder wall (2) and uterus (1). The consistent organ activity data plots were considered suitable as data input to the exponential model (EXM) module in OLINDA software (Version 1.1) to compute the time-integrated activity coefficients (TIACs) values shown in Table 2, reported as MBq-h/MBq values. The Table 2 data was comparable across the organs and tissues per participant, enabling the organ dosimetry computations. Evaluation of the Table 1 data vs. the Fig. A1 and Table 2 findings revealed that only the kidney TIACs were found to be tracer dose dependent; whereas, respective Fig. A1 SUV_max and %ID_max values were not tracer dose dependent (data not shown).

4.3. Estimates of organ radioactivity doses

The TIAC values were used as key data input to determine the estimated absorbed organ doses with OLINDA software versions 1.1 (tissue weighting factors ICRP-60) and 2.0 (tissue weighting factors ICRP-103). Both software versions and the respective tissue weighting factors were used to ensure comprehensive analyses and comparisons, as these established methods are exemplified in the field of clinical dosimetry. The resultant computed organ estimated data shown in Table 3 includes participant identification, amount of activity given, the estimated absorbed organ and remainder doses (mGy) computed by OLINDA software version 1.1 and 2.0 per participant, and similarly the Effective Dose (whole body) values reported in mSv (equivalent to mGy).

Per Table 3, the two different OLINDA software versions afforded many common output estimated organ dose values; albeit, some estimates were not computed for all the organs because of specific limitations of the respective software versions. The few Table 3 boldface entries show the paucity of elevated estimated dose values found (e.g., selective entries for the kidneys, gallbladder, thyroid, pancreas, urinary bladder wall and uterus) relative to other similar organ values. These elevated values are thought to be a result of inter-participant variability as partially reflected by the outlier organ activity data plots described above for the generation of select TIACs (e.g., Fig. A1, Appendix). Since this initial study utilized a small cohort of 8 participants, this sample size limited the statistical analyses to identify the subtle contributing factors to inter-participant variability.

Inspection of Table 3 values reveals that the estimated doses per organ are found fairly consistent among the participants per the OLINDA software version. There were no significant differences found between the Effective dose and organs that receive the highest doses (e.g., kidneys, lungs and liver). Yet, a trend is evident between injected tracer amount and effective dose for both male and female subjects. However, this relationship did not reach statistical significance due to the limited sample size. Additionally, the version 1.1 estimated dose values are somewhat less than the version 2.0 organ estimated values. This difference was not surprising based on literature precedence between the two OLINDA software versions and respective tissue weighting factors [31–33,39,41]. Statistical assessment of the Table 3 organ data vs. the Table 1 healthy control participant and injected [¹⁸F]RP-115 activity profiles revealed that the Table 3 target organ estimated values are statistically independent of participant age, the tracer radioactivity dose amount, and the mass ratio of tracer (μg)/participant (kg) values (ANOVA P-value >0.05, data not shown).

The estimated organ and remainder absorbed dose data (Table 3) was computed into composite sex values determined as mean ± SD mGy/MBq (n = 4 per sex) and Effective Dose (whole body) values reported as mSv/MBq. These data were generated to statistically group organ and effective dose estimates across the participants and allowed the comparison between the sexes, as a function of the OLINDA software versions and respective tissue weighting factors used. These comparative results are shown graphically in Fig. 4. where sex comparative p-values <0.05 (shown as *) indicate a significant distinction found between the sexes. The Fig. 4 graphs revealed that almost all the organs and tissues sampled exhibit no statistical differences between the sexes, with the exceptions of select OLINDA 1.1 data for the pancreas, adrenal glands, breasts, remainder and Effective dose, and similarly of select OLINDA 2.0 data for the adrenal glands, rectum, red marrow and eyes. These findings could be a result of organ and/or tissue size sex physiological differences. For example, for generalized average male and female sized organ and/or tissue calculations and similar tracer amounts administered, organ and/or tissue dose values are typically found higher in females because of their respective smaller size of the organ and/or tissue relative to those of men. Additionally, differential metabolic and clearance processes between males and females could be a contributing factor. The specific reasons for these estimated organ dose sex differences presently remain to be further defined.

As shown in Fig. 4 the kidneys have the highest mean absorbed mGy/mBq estimated radiation doses (e.g., range from both software versions, 0.142–0.214 mGy/mBq). Thus, the kidneys are the critical organ and are considered the primary route of activity elimination. Further support for this observation can be found in Table 3 data for Female 2 (F2^#), the only participant who did not void during the PET scanning session, shows significantly elevated estimated dose values for the kidney, urinary bladder wall and uterus compared to the other participants. This finding suggests that bladder voiding can reduce radiation exposure to the kidneys, urinary bladder, and uterus. Other prospective renal protection strategies include a lower tracer radioactivity dose (discussed below) and participant hydration levels. The liver displayed lower mean absorbed dose values across the participants (e.g., range from both software versions, 0.0392–0.0449 Gy/MBq) in comparison to the kidneys, and to a lesser extent, the small intestine. The liver and small intestine findings might suggest an influence of biliary excretion processes of the activity. Even lower levels of estimated absorbed doses (< 0.035 mGy/MBq) were found for the remaining organs and tissues evaluated, as well as the Effective dose (mSv/MBq). When the lower radioactivity [¹⁸F]RP-115 dose amounts are administered (e.g., Table 3, <200 MBq) this results in organ and Effective dose values similar to other tracers, including [¹⁸F]FDG [42–43].

4.4. Amounts of [¹⁸F]RP-115 given vs. critical kidney radioactivity estimates

This first-in-human study of the estimated absorbed organ doses following the administration of various amounts of [¹⁸F]RP-115 radioactivity and mass has afforded valuable insights into suitable injected tracer activity profiles. We have benchmarked these findings against the US Food and Drug Administration (FDA) guidance on radiation dose limits specified in publication 21CFR361 (Radiation Dosimetry Research Committee, RDRC, dose limits). This FDA guidance advises that an organ and tissue radiation dose to an adult research subject from a single study is: (a) for whole body (effective dose), active blood-forming organs, lens of the eye, and gonads a single radiation dose of 30 mSv (30 mGy, 3 rem), and (b) for other organs a single radiation dose 50 mSv (50 mGy, 5 rem).

Comparison of the Table 3 and Fig. 4 data relative to the FDA guidance thresholds reveals that: (a) all study participants showed <30 mSv (30 mGy, 3 rem) single radiation estimated doses for the effective dose (whole body), bone marrow, eye and gonads (testes and ovaries); and (b) five of the eight participants (Males 3–4, Female 1 and Females 3–4) showed <50 mSv (50 mGy, 5 rem) estimated radiation doses for the remaining organs and tissues when the tracer radioactivity given ranged between 70.3 and 189 mBq. Notably, Males 1–2 and Female 2 showed >50 mSv (50 mGy, 5 rem) elevated renal estimated radiation doses, in addition to the uterus of Female 2, when the given amount of [¹⁸F]RP-115 was between 289 and 355 MBq. Scrutiny of the ratios of the elevated estimated kidney dose to the radioactivity given, for Males 1–2 and Female 4, reveals ratios >0.21 mGy/MBq.

Although this research study did not require us to adhere to the FDA RDRC guidance for organ radiation dose threshold levels, we have aimed our initial research dosimetry studies to be at or below those FDA values that are generally recognized as safe. We appreciate that a > 50 mSv kidney radiation dose estimate is realized when giving higher radioactivity [¹⁸F]RP-115 tracer amounts (e.g., 289–355 MBq). Since additional dosimetry and related participant studies are planned at lower tracer doses, which we anticipate will alter this range of preliminary kidney dosimetry values, we appreciate that future RP-115 PET imaging studies should utilize more conservative lower tracer injected amounts, i.e., <211 MBq (e.g., 0.237 mGy/MBq × 211 MBq = 50 mGy = 50 mSv). Hence, administering <211 MBq of tracer RP-115 would be more in line with other ¹⁸F tracer injected radioactivity amounts [29–32] (e.g., range: 141 MBq – 185 MBq; typically 185 MBq) [29]. In those studies, and others employing the most commonly used tracer [^18F]FDG [42–46], suitable lower estimated radioactivity organ and tissue doses were found [42–43], and quantitative CNS PET imaging determinations were made.

5. Conclusions

Multi-step radiosynthesis and i.v. tracer [¹⁸F]RP-115 dose preparations were routinely achieved in good decay-corrected yields at suitable purity in <3 h. The tracer RP-115 was well-tolerated after i.v. administration in an initial small cohort of four male and four female healthy human volunteers. The small participation sample size of the study was a limitation to some of the statistical analyses. Initial evaluations of radioactivity biodistribution revealed good (SUV >1) radioactivity uptake and distribution into the major organs, the central nervous system and peripheral tissues. Organ and tissue time-integrated activity coefficients were computed with OLINDA software versions 1.1 and 2.0 and respective tissue weighting factors (ICRP-60 and ICRP-103) to generate estimates of absorbed organ radiation doses. Consistent organ dose estimates were found by the two computational methods across the cohort, where the kidneys were identified as the critical target organ. Depending on which computational software version and tissue weighting factors were used, statistically significant mean value estimated organ dose differences between the sexes were found for select organs and also the effective dose (whole body) values. The estimated renal radiation doses relative to the various amounts of radioactivity given defined a more optimal [¹⁸F]RP-115 tracer injected activity amount threshold of <211 MBq. This threshold is similar to tracer doses for other established clinical PET tracers and it will be used in future [¹⁸F]RP-115 human studies, including those focused on quantitative CNS tissue clinical PET imaging of neurodegeneration, such as Alzheimer’s disease and amyotrophic lateral sclerosis, among other disorders.

Abbreviations

AE

adverse events

BMI

body mass index

DICOM

digital imaging and communications in medicine

DIPEA N

N-diisopropylethylamine

EAAT2

excitatory amino acid transporter 2

ECG

electrocardiogram

EXM

exponential model

i.v.

intravenous

eIND

exploratory investigational new drug

FDA

Food and Drug Administration

FDG

fluorodeoxyglucose

¹⁸F

fluorine-18

GLT-1

excitatory amino acid transporter 2

HATU

O-(7-Aza-1H- benzotriazole-1-yl)- N, N, N′, N′-tetramethyl-uraniumhexafluorophosphonate

HC

healthy control

HIPAA

Health Insurance Portability and Accountability Act

ICRP

International Commission for Radiological Protection

IRB

Institutional Review Board

MBq

megabequerrel

mGy

milligray

MR

magnetic resonance

mSv

millisievert

L-Glu

L-glutamate

NOAEL

no-observed adverse events level

PACS

picture archiving and communicating system

PET

positron emission tomography

rem

roentgen equivalnet man

RP-115

N⁴-(7-[¹⁸F]fluoro-9H-fluoren-2-yl)asparaginate

RDRC

Radiation Dosimetry Research Committee

SAE

serious adverse events

SD

standard deviation

SPE

solid phase extraction

SUV

standardized uptake values

UCSF

University of California, San Francisco

USP

United States Pharmacopeia

VOI

volumes of interest

WB

whole body

%IA

percent injected activity

Appendix

Figure A1.

Target organ plots (as labeled), as a function of study participants (males 1–4, M1-M4; females 1–4, F1-F4) and shown as Y-axis decay-corrected percent injected activity (%IA, Y-scales are distinct per plot) per organ regions of interest vs. X-axis time (h, sampled times are the mid-time points of the WB 1–9 PET scan sessions), used to derive the respective time-integrated activity coefficients (TIACs, values per manuscript Table 2) by the curve-fitting exponential models (EXM) of OLINDA EXM software (Version 1.1). With the exception of Female 2 (F2), all of the other participants voided during the second break between scans WB8 and WB9.

Table A1.

Mean (n = 4 per sex, ± SD) estimated absorbed dose values (mGy/MBq) for target organs, the remainder and whole body (effective dose in mSv/MBq) derived from the manuscript Table 3 data with comparative vs. sex p-values shown, where organ and body portion p-values found <0.05 (shown as red) indicate a significant distinction found between the two sexes. Bar graph representations of Table A1 target organ data are shown in Fig. 4 (Panel A: OLINDA 1.1, ICRP-60; Panel B: OLINDA 2.0, ICRP-103) of the manuscript.

Panel A: OLINDA 1.1, ICRP-60 Adult Male/Female Model
Estimated Dose (mGy/MBq, mean±SD)
Organ	Male (n=4)	Female (n=4)	P-value
Adrenals	0.0181±0.00135	0.0207±0.00119	0.028
Brain	0.0135±0.00305	0.0161±0.00180	0.200
Breasts	0.00801±0.000253	0.00903±0.000633	0.024
Gallbladder wall	0.0230±0.00361	0.0219±0.00297	0.655
Heart wall	0.0214±0.00739	0.0213±0.00289	0.981
Kidneys	0.142±0.0804	0.196±0.0451	0.285
Liver	0.0392±0.0195	0.0426±0.00491	0.740
Lower, large intestine wall	0.0114±0.00246	0.0128±0.000858	0.322
Lungs	0.0446±0.0167	0.0598±0.0109	0.179
Muscle	0.0589±0.100	0.0104±0.00065	0.370
Osteogenic cells	0.0180±0.0114	0.0150±0.00144	0.628
Ovaries	-	0.0145±0.000479	-
Pancreas	0.0156±0.00485	0.0256±0.00462	0.025
Red marrow	0.0104±0.00124	0.0111±0.000499	0.345
Skin	0.0114±0.0101	0.00734±0.000677	0.453
Small intestine	0.0269±0.0137	0.0312±0.00733	0.604
Spleen	0.0340±0.0226	0.0270±0.0139	0.618
Stomach wall	0.0108±0.00294	0.0135±0.000943	0.130
Testes	0.0169±0.0202	-	-
Thymus	0.0245±0.0190	0.0255±0.00826	0.925
Thyroid	0.0178±0.0175	0.0223±0.0174	0.731
Upper, lower intestine wall	0.0142±0.00181	0.0160±0.00120	0.153
Urinary bladder wall	0.0180±0.00704	0.0588±0.0438	0.115
Uterus	-	0.0569±0.0512	-
Remainder	0.0117±0.000634	0.0134±0.000535	0.007
Effective Dose (mSv/MBq; whole body)	0.0196±0.00190	0.0262±0.00361	0.019
Panel B: OLINDA 2.0, ICRP-103 Adult Male/Female Model
Estimated Dose (mGv/MBa, mean±SD)
Organ	Male (n=4)	Female (n=4)	P-value
Adrenals	0.0386±0.0043	0.0307±0.0025	0.019
Brain	0.0132±0.0029	0.0152±0.0017	0.276
Breasts	-	0.00866±0.00067	-
Esophagus	0.0147±0.0011	0.0152±0.0006	0.484
Eyes	0.00678±0.00029	0.00797±0.00081	0.033
Gallbladder Wall	0.0248±0.0048	0.0259±0.0031	0.701
Left colon	0.0162±0.0017	0.0146±0.0007	0.128
Small Intestine	0.0341±0.0169	0.0381±0.0098	0.699
Stomach Wall	0.0163±0.0029	0.0153±0.0010	0.558
Right colon	0.0141±0.0010	0.0145±0.0009	0.541
Rectum	0.0101±0.0005	0.0157±0.0037	0.023
Heart Wall	0.0488±0.0449	0.0218±0.0027	0.275
Kidneys	0.214±0.034	0.197±0.045	0.571
Liver	0.0419±0.0082	0.0449±0.0049	0.548
Lungs	0.0363±0.0193	0.0499±0.0088	0.246
Ovaries	-	0.0144±0.0017	-
Pancreas	0.0527±0.0676	0.0244±0.0033	0.435
Prostate	0.0102±0.0003	-	-
Salivary Glands	0.00791±0.0003	0.0085±0.0008	0.220
Red Marrow	0.0101±0.0005	0.0116±0.0005	0.005
Osteogenic Cells	0.00825±0.00031	0.00899±0.00054	0.055
Spleen	0.0508±0.0194	0.0319±0.0150	0.175
Testes	0.00684±0.00028	-	-
Thymus	0.0316±0.0122	0.0284±0.0082	0.675
Thyroid	0.0121±0.0028	0.0239±0.0171	0.221
Urinary Bladder Wall	0.0183±0.0073	0.0516±0.0371	0.128
Uterus	-	0.0573±0.0514	-
Effective Dose (mSv/MBq; whole body)	0.0179±0.0018	0.0223±0.0033	0.060