Overall Gadolinium Exposure Within the First 5 Months After Injection of Human Equivalent Doses of Gadopiclenol, Gadoterate, or Gadobutrol in Healthy Rats

Abstract

Introduction:

Gadopiclenol is a high relaxivity macrocyclic and nonionic Gadolinium-Based Contrast Agent (GBCA) for central nervous system (CNS) and Body MRI, approved in September 2022 by the Food and Drug Administration and in December 2023 by the European Medicine Agency and others European health authorities. Gadopiclenol is currently indicated at half gadolinium-dose (0.05 mmol/kg body weight) compared to the other nonspecific marketed GBCAs. This study aims to evaluate the impact of this gadolinium (Gd) dose reduction in terms of overall Gd body exposure. Requiring tissue samples at different times, this information is only accessible through animal experiment. In this study, the Gd exposure over a 5-month period was evaluated in healthy rats after a single injection of gadopiclenol in comparison with 2 other macrocyclic GBCAs approved for human use, gadobutrol and gadoterate, all administered at their respective human equivalent dose (HED).

Material and Methods:

Healthy 9-week-old female Sprague-Dawley rats were randomly allocated to 4 groups, receiving 1 single intravenous injection of gadoterate (Dotarem, 0.6 mmol/kg), gadobutrol (Gadovist, 0.6 mmol/kg), gadopiclenol (Elucirem, 0.3 mmol/kg), or saline (control group). Animals were euthanized 1 day (D1), 1 week (W1), 1 month (M1), or 5 months (M5) after the injection (n = 10/group and time-point). Selected tissues (including central [CNS] and peripheric nervous system [PNS] organs, excretion organs, bone) were collected for subsequent total Gd determination with inductively coupled plasma mass spectrometry. Based on Gd concentration measurements at these different time points, the 5 months overall exposure to Gd in each organ was estimated by calculating the area under the curve (AUC), between the first and the last time point. The whole study was performed in a blinded manner.

Results:

Following gadopiclenol administration to rats at the HED, overall Gd exposure over 5 months was found to be 25% to 40% lower compared to gadoterate and gadobutrol, respectively. Organ by organ, Gd exposure reduction is observed over the studied period in the plasma, CNS (cerebellum, cortical brain, subcortical brain, brain stem, spinal cord), PNS (spinal nodes, sciatic nerve, footpads), the spleen, the skin, the liver, and the kidney. In the femur, the Gd exposure after gadopiclenol administration was higher or equivalent compared to gadobutrol and gadoterate in the mineral parts of the bone (diaphysis and epiphysis), but lower in the bone marrow. Residual Gd found in each tissue studied is extremely low relative to the injected dose), with values ranging from 10⁻⁶ (CNS) to 10⁻³ (kidney, mineral bone) % injected dose/g of organ.

Conclusions:

Under our experimental conditions, the overall measured Gd exposure over 5 months following gadopiclenol injection in rats at the HED is 25–40% lower than that after gadoterate and gadobutrol injections, respectively.

Gadolinium-based contrast agents (GBCAs) are widely used to enhance the quality of magnetic resonance imaging (MRI) images, providing critical information for accurate diagnosis. During the past years, studies have shown that Gd can be detected in the brain and other tissues, not only in patients with advanced kidney disease but even in individuals with normal kidney function. Indeed, research has shown that after administration, the Gd was not eliminated completely from the body. The first concern regarding the safety of GBCAs was the description of nephrogenic systemic fibrosis (NSF) pathology in renally impaired subjects in 2006.¹ This was followed by the finding of GBCA-induced brain hypersignals in patients with normal renal function in 2015.² A few years later, attention shifted to the global retention of Gd in the whole body.³ Recently, the American College of Radiology's Committee on Drug and Contrast Agents proposed a classification of a group of symptoms appearing sometimes in patients after the administration of GBCA, with undefined causal relationship. These symptoms were described and reported as SAGE—Symptoms Associated with Gadolinium Exposure.⁴ They include, among others, headache, bone and joint pain, joint stiffness, cloud mentation, and peripheral neuropathic pain. There is also a suspected link between Gd retention and the occurrence of the small fibers neuropathy (SFN), a pathology affecting small nerve fibers responsible for pain and temperature sensations.⁵ Patients with SFN often experience symptoms like burning, pain, tingling, and numbness. Some researchers propose that Gd deposition in peripheral nerves could contribute to the development or exacerbation of SFN,⁶ although some studies contradict each other,⁷ and more studies are required to establish a direct potential causal relationship.

These confirmed or potential safety issues, especially with linear GBCAs, are questioning the dose of injection. In the clinical routine, due to their established higher stability, macrocyclic GBCAs are favored over linear ones because they are considered safer and less likely to release gadolinium ions into the body.⁸

The exact mechanisms and long-term effects of Gd retention are not fully elucidated, but studies have confirmed that Gd can be retained in preferential tissues, mostly for linear GBCAs with presence of Gd not complexed to its initial ligand and to a lesser extent for macrocyclic GBCAs, mainly as intact GBCA.

Gadopiclenol (Elucirem/Vueway) is a new macrocyclic and nonionic GBCA recently approved by the Food and Drug Administration (FDA) and European Medicine Agency (EMA) and in some other European countries for CNS and Body MRI, in adults and children over 2 years old (except in Switzerland, in adults only). Gadopiclenol has the highest relaxivity among marketed nonspecific agents (12.8 mM⁻¹.s⁻¹ versus 3.6 mM⁻¹.s⁻¹ for gadoterate in human serum at 1.5 T) and presents the best kinetic stability in acidic media compared with all commercial GBCAs.^9,10 Gadopiclenol is indicated at half Gd-dose compared to the standard dose of other marketed nonspecific GBCAs, with at least equivalent diagnostic performances showed in different models or pathologies both in preclinical studies^11–13 and in clinical trials.^14–19 The safety of gadopiclenol was assessed and resulted to be very good as well in preclinical^20–22 and clinical studies.^{15–17,23–27} However, benefits of the Gd dose reduction in term of overall long-term Gd exposure, meaning the evolution of the Gd load in different organs after injection, are not documented to date as it requires tissue samples at different times. Nevertheless, this information is accessible through animal experiment.

The aim of this study is to evaluate the time-dependent wash-out and the resulting exposure of Gd in various body organs in rats after a single injection of gadopiclenol at the approved dose, which corresponds to half the standard Gd dose, compared to gadoterate and gadobutrol.

MATERIALS AND METHODS

All experimental procedures were performed in accordance with the in-house Animal Welfare Ethics Committee, French regulations, and in compliance with the European Economic Community Directive (2010/63/EU) on animal welfare.

Animal Model and Administration Protocols

Ten female Sprague-Dawley rats were included per group (3 studied GBCAs and one control group) and euthanasia time point (D1, W1, M1, M5 corresponding to 1 day, 1 week, 1 month, and 5 months after injection). Therefore, the total number of animals was 40 per time point, and 160 for the whole study. The animals were 9 weeks old at the time of the administration. To prevent Gd contaminations between GBCAs by urine, blood or feces, the animals were housed by groups. A single injection of GBCA or saline at the HED, corresponding to 6-fold the human dose (FDA guidelines),²⁸ has been applied under gaseous anesthesia with isoflurane (induction at 4%, injection at 2.5%): 0.6 mmol/kg for gadoterate (1.2 mL/kg) and gadobutrol (0.6 mL/kg) and 0.3 mmol/kg for gadopiclenol (0.6 mL/kg), administered in the tail vein with a 24G BD Insyte catheter. Saline was injected at 1.2 mL/kg as for the gadoterate group. Injections were performed blindly group per group to prevent Gd contamination between groups, in random order. Sliding sheets were placed on the scale, the injection surface, and the induction cage. Only single-use material was applied.

Blood and Skin Sampling During the Study

The injection and sampling scheme are summarized in Figure 1. Most of the tissues were sampled at 1 day (D1), 1 week (W1), 1 month (M1), or 5 months (M5) after the injection, at completion of the chosen washout period. However, blood and skin were sampled regularly all along the study before sacrifice. Blood was sampled under slight gaseous anesthesia in the W1 group only at the following time points: 3.5 hours after injection and then 1, 2, 3, 4, 5, and 7 days after injection for animals of W1 group. Approximately 100 μL of blood was collected by day in heparin-tubes from the sublingual vein. All samples were gently swirled and rapidly placed at +4°C until centrifugation. Around 50 μL of plasma was collected for Gd determination. Dorsal skin was repeatedly sampled under gaseous anesthesia with sterile and unique device in the W1 groups, at 1, 3, 5, and 7 days after the GBCA administration and every month in the M5 group, from M1 to M5 (except M4). For this procedure, animals received an injection of buprenorphin at 0.05 mg/kg by subcutaneous route (0.17 mL per 100 g of bodyweight, of a 1:10 dilution of the initial solution 0.3 mg/mL) at least 30 minutes before sampling. Additionally, 0.1 mL of Xylocaïne-adrenaline (20 mg–0.005 mg) solution was applied locally intra-dermis in the sampling area, 10–15 minutes before sampling. The sample consisted in a 6 mm-diameter-disk, realized thank to a biopsy punch under gaseous anesthesia. Two stitches closed the biopsy area, with a nonresorbable suture thread. Samples were placed at −20°C until dosing. If needed, the buprenorphin administration (0.05 mg/kg) was repeated 6–8 hours after the biopsy and/or on the next morning, or when the rat got rid of the stiches and new one(s) should be done.

FIGURE 1.

Study scheme (H3.5: sampling at 3.5 h after injection, D: days, M: months) with all the sampling times for all tissues.

Euthanasia and Other Organs Sampling

Euthanasias at 1 day (D1), 1 week (W1), 1 month (M1), or 5 months (M5) after the injection were performed under gaseous anesthesia (5% Isoflurane) and a terminal blood sampling was done at the level of the cava vein. To prevent Gd contamination between groups, euthanasias were performed group per group, in random order. As done during the injections, sliding sheets were placed on the sacrifice surface and in the induction cage. Most of the time, single use material was used and surgical material, when needed. Surgical tools were washed out with a 0.01 M EDTA solution between each group to eliminate eventual Gd traces. The following organs were sampled for: CNS (cerebellum, cortical and subcortical brain, brainstem), femur (diaphysis, epiphysis, bone marrow), kidney (cortex and medulla), skin, spleen, liver, plasma, and organs of interest in the context of small fiber neuropathy (“SFN organs”: sciatic nerve, footpads, dorsal ganglia root (DRG, or spinal node), spinal cord). SFN organs were not sampled at D1.

Measurement of Elemental Gadolinium Tissue Concentrations

Elemental Gd concentrations were measured in biological samples by inductively coupled plasma mass spectrometry (ICP-MS) (7700x; Agilent Technologies, Santa Clara, CA) after sample mineralization in 65% nitric acid (HNO₃) with a dilution factor varying between 5 and 18.8 (w/v), for 8 hours at 80°C. The mineralized samples were diluted into final 6.5% HNO₃ solution prior to the ICP-MS analysis. In this last step, inorganic Indium (In) was added as internal standard. The ICP-MS instrument was daily checked using a multielement solution. External calibration curve of inorganic Gd in 6.5% HNO₃ combined with internal standardization were used by monitoring the response of the ¹⁵⁸Gd and ¹¹⁵In isotopes. Depending on the organ and time point, the quantification range of the method was either 0.05–100 μg/L or 0.01–10.0 μg/L in solution. The lower limit of quantification (LLOQ) can vary for each series of analysis; therefore, LLOQ is specified in the graphs for each matrix. Results are expressed in nmol Gd/g of wet tissue (tissue samples) or nmol Gd/L of plasma and were calculated by multiplying the Gd concentration measured in the final solution by the dilution factors applied during the sample preparation.

Data Treatment and Statistical Analyses

Values inferior to the LLOQ were arbitrarily replaced by LLOQ for calculation of the means, standard deviations (SD), and statistical analyses. The potential aberrant data was detected thank to the ROUT test (GraphPad Prism) at a 5% risk. Then, the decision to exclude or to keep the value was based on an observation of the graphs, and if the value is consistent with values from the precedent time point, if existing.

The normality was assessed with the help of D'Agostino & Pearson test. If normality test was passed, the equadisticity of the data was checked by Brown-Forsythe test. If equadisticity was respected, an ordinary One-way ANOVA & Tukey comparisons tests were performed. If the data did not respect equadisticity, Brown-Forsythe and Welch ANOVA & Dunnet's T3 multiple comparisons tests were performed. If the test of normality resulted negative (always the case for saline group, present at W1-M1-M5), a Kruskal-Wallis & Dunn's multiple comparison tests were performed to compare the groups. On graphs, a single star (*) indicates a P value <0.05, 2 stars (**) indicates a P value <0.01, and 3 stars (***) indicates a P value <0.001.

Global Gd Exposure and Area Under the Curve Calculation

Concentrations data C(t) in the organ of interest at the time point t were fitted with the following equations, corresponding to a 1-phase decay model: C(t) = (Y ₀−P)·e ^−k·t +P. The estimated parameters are: Y₀ is the maximum Gd concentration value; P is the plateau to which the value tends to infinity; k is the rate constant equal the reciprocal of the X axis units. The Gd global exposure in each organ was then estimated through the calculation of the AUC (area under the curve) between 2 time points, t₁ and t₂, calculated as following: $\mathit{AUC}\left(t_1{t}_2\right)=\frac{Y_0-P}{k}·\left(e^{\left(-k.t_1\right)}-e^{\left(-k.t_2\right)}\right)+P·\left(t_2-t_1\right)$

RESULTS

All the animals completed the study (no mortality). Gd distribution and elimination was plotted based on the concentrations evolution (nmol/g) for all products in each sampled organ, at all time points (Figs. 2–8). Then, fitting of the time-dependent mean Gd concentrations was performed by a 1-phase decay model to calculate the 5-month Gd exposure. Example of fitting is shown on Figure 9 (cerebellum data). All the fitted parameters and the resulting organ-by-organ exposure calculation are summarized on Table 1. The areas under the curves (AUCs) over the studied period representing the overall Gd exposure after administration of GBCAs are expressed as pie charts in Figure 10. The differences of exposure organ per organ versus gadopiclenol are plotted in (Fig. 11). All organs are discussed one-by-one in the following sections.

FIGURE 2.

Plasma Gd concentrations. LLOQ = 0.03 nmol/g at H3.5, and LLOQ = 0.02 nmol/g for D1-M5.

FIGURE 8.

Individual Gd concentrations measured in the SFN organs at different time points. LLOQ = 0.003 nmol/g at all time points and for all organs.

FIGURE 9.

Modeling of the tissue exposure. Example of fitting of data on cerebellum concentrations with the formula C(t) = (Y ₀−P) · e ^−k·t +P, and equation for area under the curve (AUC) calculation.

TABLE 1.

Fitting Parameters With the Formula C(t) = (Y ₀−P) · e ^−k·t +P, and Area Under the Curve Calculation

Organs		Contrast Agent	R 2	Fitted Parameters			AUC (nmol/g*Weeks)
				T 1/2  = Ln2/k (Weeks)	Y0 (nmol/g)	P (nmol/g)
TOTAL all organs		Gadoterate	NA	NA	NA	NA	613
		Gadobutrol	NA	NA	NA	NA	761
		Gadopiclenol	NA	NA	NA	NA	459
Plasma		Gadoterate	0.89	0.05	1.495	0.09017	0.163
		Gadobutrol	0.63	0.04	3.465	0.08147	0.191
		Gadopiclenol	0.64	0.04	1.944	0.03388	0.101
Central nervous system	Cerebellum	Gadoterate	0.95	0.5	0.3694	0.01300	0.458
		Gadobutrol	0.93	0.4	0.3927	0.01806	0.511
		Gadopiclenol	0.85	0.4	0.1418	0.01395	0.340
	Subcortical brain	Gadoterate	0.95	0.5	0.3227	0.01430	0.469
		Gadobutrol	0.94	0.4	0.2997	0.01409	0.411
		Gadopiclenol	0.88	0.4	0.1439	0.01149	0.296
	Cortical brain	Gadoterate	0.95	0.5	0.282	0.01526	0.456
		Gadobutrol	0.92	0.4	0.3121	0.01807	0.481
		Gadopiclenol	0.88	0.4	0.1166	0.00808	0.215
	Brain stem	Gadoterate	0.96	0.5	0.309	0.01466	0.460
		Gadobutrol	0.92	0.5	0.3078	0.01226	0.403
		Gadopiclenol	0.91	0.5	0.1256	0.00662	0.208
Skin		Gadoterate	0.90	0.3	8.549	0.04302	3.09
		Gadobutrol	0.73	0.2	16.55	0.24920	7.17
		Gadopiclenol	0.86	0.3	3.858	0.05733	2.32
Excretion/elimination organs	Kidney cortex	Gadoterate	0.95	0.6	464.9	4.509	434.7
		Gadobutrol	0.95	0.6	567.7	4.375	531.5
		Gadopiclenol	0.89	0.7	290.1	2.159	296.1
	Kidney medulla	Gadoterate	0.95	0.5	121.4	2.750	117.0
		Gadobutrol	0.92	0.5	142.2	3.627	145.3
		Gadopiclenol	0.84	0.4	83.4	2.863	94.3
	Liver	Gadoterate	0.96	0.4	15.45	0.0606	7.57
		Gadobutrol	0.94	0.4	19.10	0.0734	10.16
		Gadopiclenol	0.96	0.6	9.51	0.0287	7.51
Spleen		Gadoterate	0.95	0.5	14.65	0.1062	10.65
		Gadobutrol	0.98	0.4	16.86	0.2143	12.11
		Gadopiclenol	0.97	0.5	8.476	0.1168	7.34
Bones	Femur diaphysis	Gadoterate	0.76	0.2	1.785	0.5547	11.31
		Gadobutrol	0.66	0.1	3.1	0.8709	17.53
		Gadopiclenol	0.62	0.5	2.476	0.9757	20.32
	Femur epiphysis	Gadoterate	0.94	0.4	4.693	0.5349	12.49
		Gadobutrol	0.96	0.4	6.467	0.8286	18.68
		Gadopiclenol	0.74	0.6	4.712	0.7873	18.47
	Bone marrow	Gadoterate	0.96	0.4	8.879	0.2931	9.48
		Gadobutrol	0.93	0.4	9.423	0.3877	11.50
		Gadopiclenol	0.94	0.4	5.187	0.2557	7.44
Peripheric nervous system	Footpads	Gadoterate	0.82	0.7	1.613	0.0056	0.640
		Gadobutrol	0.93	0.3	5.888	0.0203	0.726
		Gadopiclenol	0.74	0.5	2.091	0.0047	0.453
	Sciatic nerve	Gadoterate	0.83	0.8	3.875	0.0099	1.869
		Gadobutrol	0.77	0.8	3.680	0.0114	1.809
		Gadopiclenol	0.82	1.0	1.528	0.0080	1.293
	Spinal nodes (DRG)	Gadoterate	0.91	0.8	2.631	0.056	2.293
		Gadobutrol	0.81	0.8	2.325	0.085	2.590
		Gadopiclenol	0.94	1.1	1.193	0.040	1.770
	Spinal cord	Gadoterate	0.85	1.4	0.1715	0.007	0.327
		Gadobutrol	0.60	1.1	0.2446	0.005	0.309
		Gadopiclenol	0.72	1.4	0.0881	0.003	0.159

AUC, area under the curve.

FIGURE 10.

Values of the Areas Under the Curve (in nmol/g*w) of the different organs for gadoterate, gadobutrol or gadopiclenol. Surfaces are proportional to the exposure. Overall surface of gadopiclenol is 25% lower than gadoterate and 40% lower than gadobutrol.

FIGURE 11.

Difference in overall Gd exposure between gadopiclenol and gadoterate or gadobutrol over 5-month period.

Plasma

At 3.5 hours after injection, corresponding to around 10 half-lives of the GBCAs, the Gd plasma concentration is around 2 nmol/g, varying between 1.32 ± 0.94 nmol/g for gadopiclenol, the lowest concentration, and 2.30 ± 1.62 nmol/g for gadobutrol, the highest (Fig. 2). While no strict 2-fold reduction in Gd concentration is found for gadopiclenol compared to the other GBCAs at this time point (H3.5), this is observed for residual circulating Gd between 24 and 48 hours (D1 and D2, but not statistically different within GBCA groups). At D3, circulating Gd is around 3 times lower for gadopiclenol than for gadobutrol and gadoterate. However, concentrations for saline group are at the same level, suggesting a possible contamination at this time point. At D5, all Gd concentrations in gadopiclenol group are equal or inferior to the LLOQ, while those in gadoterate are around 6 times higher, and surprisingly higher than the day before. One week after the injection, residual circulating Gd is too low to be quantifiable, whatever the group.

Gd global exposure was calculated between H3.5 and W1, as the values of the following time points were below the LLOQ. Gadopiclenol, injected at half-dose combined with a fast clearance from blood, resulted in the lowest Gd exposure: the corresponding AUC was of 0.101 nmol/g*w (ie, nmol/g*week), vs 0.163 nmol/g*w in gadoterate (+61%) and 0.191 nmol/g*w in gadobutrol (+89%).

CNS Structures

In the CNS structures (cerebellum, subcortical brain, cortical brain, and brainstem) at D1 (Fig. 3), Gd concentration measured is around 0.25–0.35 nmol/g for the gadoterate and gadobutrol, and at least twice lower for gadopiclenol, around 0.10–0.12 nmol/g, leading to a significant difference compared to the other groups (P < 0.05–P < 0.001 depending on the structure). One week after injection (W1), Gd concentrations for gadopiclenol are still inferior by a factor of 2, but sometimes (subcortical brain and brainstem) statistically significant only compared to gadoterate. At M1, Gd concentrations for gadopiclenol are generally twice inferior to gadoterate and gadobutrol, even inferior to the lower limit of quantification (0.02 nmol/g) in majority in the cortical brain. In the cerebellum at M1, the situation is different: Gd concentrations are similar between groups, and inferior to the saline group, suggesting a possible contamination. At M5, Gd concentrations are either inferior or very close to the LLOQ of 0.003 nmol/g, showing a total washout of the GBCAs. However, the half-time of concentration decrease over the studied period is approximately half a week (~0.4–0.5 week) for the 3 products tested and all CNS tissues (Table 1). Gadopiclenol injected at half Gd-dose, combined with a good excretion in the CNS structures, leads to the lowest Gd exposure between D1 and M5 (AUC of 0.208–0.340 nmol/g*w depending on the structure), while the overall exposure of gadoterate and gadobutrol is more than doubled in cortical brain and brainstem vs gadopiclenol (AUC of +112% to 124% and of +94% to +121%, respectively), and increased to a lesser extent in cerebellum and subcortical brain (AUC of +35% to +50% and +39% to +58% respectively, leading to +71–74% in mean in the 4 parts of the CNS).

FIGURE 3.

Individual Gd concentrations measured in the CNS organs at the different time points. LLOQ = 0.02 nmol/g at D1-W1, and LLOQ = 0.003 nmol/g at M1-M5.

Skin

In the skin (Fig. 4), the sampling of biopsies allows to have a thin follow-up during the first week, and between M1 and M5. Gd concentrations at D1 are significantly decreased in gadopiclenol group compared to gadobutrol by a factor of 3 (P < 0.05) and to gadoterate, by a factor of 2 (P < 0.05). During the first week post injection (D3-W1), Gd concentrations decreased for all groups. Gd elimination is slower for gadopiclenol compared to the other tested GBCAs, resulting in similar or lower Gd concentrations in gadoterate and gadobutrol groups 1 month after injection (M1). From M2, the majority of the Gd concentrations reach the lower limit of quantification, hence, the AUC was calculated until M2 only. The washout in the skin is therefore complete, with shorter half-lives as compared to CNS (~0.2–0.3 weeks).

FIGURE 4.

Individual Gd concentrations measured in the skin at different time points. LLOQ = 0.1 nmol/g at D1-W1, and LLOQ = 0.03 nmol/g at M1-M5.

Regarding the AUC in the skin over the D1-M2 period, gadopiclenol showed the lowest overall Gd exposure (2.32 nmol/g*w), followed by gadoterate (3.09 nmol/g*w, +33% vs gadopiclenol) and gadobutrol with a doubled AUC (7.17 nmol/g*w, +209% vs gadopiclenol).

Kidney

Kidney Cortex

In the kidney cortex (Fig. 5A), Gd concentrations measured 1 day after injection are between 253 ± 66 nmol/g and 489 ± 70 nmol/g in gadopiclenol and gadobutrol groups, respectively (P < 0.001). A ratio of 2 is observed between Gd concentrations of these 2 groups, but not between gadopiclenol and gadoterate (398 ± 70 nmol/g, P < 0.001). Then, an important washout is observed in all groups, with Gd concentrations decreasing by a factor of 2–3 at W1 (between 109 ± 32 nmol/g for gadopiclenol, and 194 ± 65 nmol/g for gadobutrol), then further reduced 10-fold by M1 (9.8 ± 2.1 nmol/g for gadopiclenol, 16.4 ± 6.0 nmol/g for gadobutrol), ultimately or eventually dropping below 0.4 nmol/g at M5. The washout in the kidney cortex is massive between D1 and M5: more than 99.94% is eliminated, whatever the group.

FIGURE 5.

Individual Gd concentrations measured in the kidney cortex and medulla at different time points. LLOQ = 0.2 nmol/g at D1-W1, and LLOQ = 0.02 nmol/g at M1-M5.

The overall exposure estimated by the AUC between D1 and M5 in the kidney cortex is 296.1 nmol/g*w for gadopiclenol, followed by gadoterate (434.7 nmol/g*w, +47% vs gadopiclenol) and gadobutrol (531.5 nmol/g*w, +80% vs gadopiclenol).

Kidney Medulla

In the kidney medulla (Fig. 5B), Gd concentrations measured at D1 are around 4 times lower than in the cortex: between 66 ± 24 nmol/g for gadopiclenol, and 116 ± 25 nmol/g for gadobutrol (P < 0.001 vs gadobutrol; P < 0.05 vs gadoterate). The ratio between Gd concentrations of gadopiclenol and the other GBCAs tested ranges from 1.50 to 1.76. As in kidney cortex, an important washout is observed in all groups (at least −98.9% between D1 and M5), yet the Gd concentrations measured at M5 are higher than those found in the cortex. Gd excretion in gadopiclenol group is slightly slower, as Gd concentrations are the lowest at D1 but the highest at M5 (even if low, with 0.72 ± 0.12 nmol/g, vs 0.28 ± 0.09 nmol/g for gadoterate and 0.44 ± 0.23 for gadobutrol).

The AUC between D1 and M5, representing the overall exposure, in the kidney cortex is 94.3 nmol/g*w for gadopiclenol, followed by gadoterate (117.0 nmol/g*w, +24% vs gadopiclenol) and gadobutrol (145.3 nmol/g*w, +54% vs gadopiclenol).

Summary of Gd in Kidney

In summary, the Gd concentrations in the kidney are about 4 times higher in the cortex compared to the medullar part. The half-time of concentration decrease over the period is also slightly shorter in the medulla (0.4–0.5 weeks vs 0.6–0.7 weeks in the cortex), leading to 3–4 times higher Gd exposure over the D1-M5 period in the kidney cortex.

Bone Structures

Diaphysis

In the femur diaphysis (Fig. 6A), Gd concentrations measured at D1 are rather low, from 1.38 ± 0.30 nmol/g for the lowest, gadoterate, to 2.22 ± 0.77 nmol/g for the highest, gadopiclenol (P < 0.05 vs gadoterate). There is a slight decrease in Gd concentrations between D1 and W1, ranging from −38 to −55%. However, from W1 to M5, the Gd concentrations remain generally stable, with poor washout, even slightly rising between M1 and M5, except for gadopiclenol. Whatever the time point, gadopiclenol is the group showing the highest Gd concentration even if low (<1 nmol/kg). It also exhibits the longest half-time of concentration decrease (0.5 weeks, compared to 0.1–0.2 weeks for gadobutrol or gadoterate). With regard to the global exposure, due to the slow washout, Gd remain detectable for all GBCAs. Gadopiclenol shows the highest AUC, with 20.32 nmol/g*w, followed by gadobutrol (−14% vs gadopiclenol) and gadoterate the lowest with 11.31 nmol/g*w (−44% vs gadopiclenol). It is worth noting that the model's coefficient of determination is not high (eg, R² = 0.62 for gadopiclenol in diaphysis).

FIGURE 6.

Individual Gd concentrations measured in the femur diaphysis, epiphysis and bone marrow at different time points. LLOQ = 0.1 nmol/g at D1-M1, and LLOQ = 0.2 nmol/g or 0.02 nmol/g at M5.

Epiphysis

In the femur epiphysis (Fig. 6B), Gd concentrations measured at D1 are more than doubled compared to the diaphysis: 3.78 ± 0.68 nmol/g for the lowest, gadoterate, 4.11 ± 1.57 for gadopiclenol, and 5.11 ± 0.49 nmol/g for the highest, gadobutrol (P < 0.01 vs gadoterate). The decrease in Gd concentrations between D1 and W1 ranges from −68% for gadobutrol, to −52% for gadopiclenol. This slightly slower washout for gadopiclenol leads to higher Gd concentrations in this group from W1 compared to the other GBCAs. Unlike the diaphysis, there is still significant washout between W1 and M1, ranging from −37% to −59%. However, from M1 to M5, the Gd concentrations remain generally stable, except in the gadopiclenol group, which shows a slight decrease, ending to a lower mean concentration than the gadobutrol group at M5.

The overall exposure in femur epiphysis is in the same range than in diaphysis, with an AUC around 18.47 nmol/g*w for both gadobutrol and gadopiclenol, and 12.49 nmol/g*w for gadoterate (−32% vs gadopiclenol or gadobutrol).

Bone Marrow

In the femur bone marrow (Fig. 6C), gadopiclenol exhibits the lowest Gd concentrations at D1, averaging at 4.15 ± 0.78 nmol/g, while the other 2 groups show a 67–77% increase in Gd concentrations compared to gadopiclenol (P < 0.01 or lower vs gadopiclenol). One week later, the gadopiclenol Gd concentration is 1.20 ± 0.22 nmol/g. From M1, there is no clear difference between the groups, with a good washout by M1 (−93 to −96% vs D1) but no longer between M1 and M5, with values still quantifiable at this late time point, up to 0.4 nmol/g across all groups.

In the bone marrow, the Gd exposure over the D1-M5 period is lowest for gadopiclenol at 7.44 nmol/g*w, followed by gadoterate (+27% vs gadopiclenol) and gadobutrol at 11.50 nmol/g*w (+55% vs gadopiclenol). The half-time of concentration decrease is within the same range across all GBCAs (~0.4 week).

Summary of Gd in Bone Structures

In the femur, the Gd primarily localizes at short term in the bone marrow, followed by the epiphysis and least in the diaphysis. However, the washout is reversed, with an effective Gd elimination from the bone marrow but poor removal from the diaphysis. In the case of gadopiclenol injected at half the gadolinium-dose, the Gd pattern differs from other GBCAs. More Gd is retained in the diaphysis, equivalent in the epiphysis, and less in the bone marrow, both at short and long term. Overall with gadopiclenol, the Gd exposure is increased (or rather close to the one in gadobutrol) in the epiphysis and diaphysis, yet it is reduced in the bone marrow.

Liver

In the liver (Fig. 7A), Gd concentrations measured at D1 are the lowest in gadopiclenol group, with 8.1 ± 1.1 nmol/g (P < 0.01 or lower vs the other groups), and the highest in gadobutrol group with 15.1 ± 3.2 nmol/g. The washout is important between D1 and W1 (−63 to −82%), and nearly total by M1 (−98.4–99.1% vs D1), with Gd concentrations below 0.13 nmol/g. At M5, the residual Gd is close to the limit of quantification of 0.02 nmol/g (washout higher than 99.7% vs D1). Gadopiclenol exhibits the slowest Gd washout between D1 and W1 (−63%), leading to the second highest Gd concentrations at both W1 and M1 after gadobutrol.

FIGURE 7.

Individual Gd concentrations measured in the liver and spleen at different time points. LLOQ = 0.1 nmol/g at D1 in liver, and LLOQ = 0.02 nmol/g at W1-M5 and in spleen.

The overall Gd exposure in the liver is the lowest with gadopiclenol at 7.51 nmol/g*w. Gadoterate is almost similar to gadopiclenol, while gadobutrol has the highest exposure (10.16 nmol/g*w, +35% vs gadopiclenol and gadoterate).

Spleen

In the spleen (Fig. 7B), Gd concentrations after gadopiclenol injection are the lowest at D1, with 7.02 ± 0.90 nmol/g. They are twice higher in the other GBCA groups (P < 0.005 vs gadopiclenol). Gadopiclenol shows a slowest Gd elimination, leading to similar Gd concentrations to gadoterate at M1 and M5 (respectively around 0.194 ± 0.030 nmol/g at M1 and 0.077 ± 0.011 nmol/g at M5). Despite a good washout in this tissue (−99.4 to −99.7% vs D1), Gd concentrations remain above the LLOQ at M5.

Gadopiclenol has the lowest Gd overall exposure in the spleen at 7.34 nmol/g*w, followed by gadoterate 10.65 nmol/g*w (+45%) and gadobutrol at 12.11 nmol/g*w (+65%)

Summary of Gd in the Liver and Spleen

In the liver and spleen, the Gd concentrations measured at D1 are around 12–15 nmol/g for both gadoterate and gadobutrol, while for gadopiclenol the concentration is reduced by 32–48% with Gd levels around 7.5 nmol/g. The washout is rather good, leading to similar Gd concentrations from 1.5 to 3.0 nmol/g at W1 for all GBCAs, and close to the LLOQ at M5 in the liver, not in the spleen (still around 0.1–0.2 nmol/g). The AUC over the period for gadopiclenol is 7.3 nmol/g*w, lower than other GBCAs in the spleen (+45–65%), but similar to gadoterate in the liver.

Peripheral Nerve System Organs

As the organs of the peripheral nervous system were not sampled at the D1 time point, the overall exposure was estimated by the calculation of the AUC between 1 week (W1) and 5 months (M5), which is thus less representative with no early time point as for the other organs (Fig. 8).

Footpads

Gd concentrations in the footpads (Fig. 8A) are rather low and homogeneous between the groups at W1, comprised between 0.516 ± 0.263 nmol/g (for gadopiclenol) and 0.736 ± 0.165 nmol/g (for gadobutrol) (no significant difference). A significant washout is observed between W1 and M1, with the lowest mean Gd concentration at 0.012 ± 0.005 nmol/g for gadopiclenol, and the highest at 0.030 ± 0.017 nmol/g for gadoterate (P < 0.05 vs gadopiclenol). However, the control group values are in the same range, and none below the LLOQ, suggesting a contamination, hence lowering the confidence in the data. Five months after the GBCA administration, the Gd concentrations are very low (close to the LLOQ of 0.003 nmol/g) for gadoterate and gadopiclenol (washout of −99% vs D1), yet still quantifiable, while mean Gd concentrations for gadobutrol are around 5 times higher (washout of −97% vs D1), even increased compared to 1 month after administration (M1).

With regard to the overall exposure estimated by the area under the curve between one week and 5 months after injection of GBCAs, gadopiclenol shows the lowest Gd exposure, with an AUC of 0.453 nmol/g*w, followed by gadoterate (0.640 nmol/g*w, +41%) and gadobutrol (0.726 nmol/g*w, +60% vs gadopiclenol).

Sciatic Nerve

In the sciatic nerve (Fig. 8B), Gd concentrations are the lowest at W1 for gadopiclenol, averaging 0.78 ± 0.29 nmol/g, while gadoterate and gadobutrol show doubled Gd concentrations (1.55 ± 0.58 and 1.48 ± 0.66 nmol/g, respectively). Afterwards, the Gd concentrations are equivalent across the different groups, around 0.10–0.11 nmol/g at M1, and around 0.008–0.011 nmol/g at M5, close to the LLOQ of 0.003 nmol/g (−99% of washout vs W1).

The overall exposure over the W1-M5 period is the lowest for gadopiclenol (1.293 nmol/g*w), followed by both gadobutrol and gadoterate respectively 1.809 and 1.869 nmol/g*w (+40–45% vs gadopiclenol).

Spinal Nodes (Dorsal Root Ganglia)

In the spinal nodes (Fig. 8C), gadopiclenol shows the lowest Gd concentration at W1 (0. 663 ± 0.123 nmol/g), followed by gadobutrol (0.980 ± 0.358 nmol/g) and gadoterate the highest (1.130 ± 0.282 nmol/g). At M1, gadopiclenol is similar to gadoterate and gadobutrol. However at M5, gadopiclenol shows the lowest residual Gd, while remaining above the LLOQ (washout between −91.3% for gadobutrol and −95.0% for gadoterate).

Gadopiclenol exhibits the lowest overall Gd exposure, at 1.770 nmol/g*w. Gadoterate and gadobutrol both show a greater Gd exposure, at 2.293 (+30%) and 2.590 nmol/g*w (+46%), respectively.

Spinal Cord

In the spinal cord (Fig. 8D), the Gd concentrations are quite low. Gadopiclenol has the smallest Gd concentration at all time point, starting with 0.054 ± 0.025 nmol/g at W1, and hitting the LLOQ (0.003 nmol/g) at M5. The washout in the other groups is rather good (−93% for gadoterate to −96% for gadobutrol vs W1), although most values remain quantifiable at the later time point.

The overall Gd exposure over the W1-M5 period in the spinal cord is 0.159 nmol/g*w for gadopiclenol and the double for the other GBCAs (around 0.320 nmol/g*w, +94% to +106%).

Summary of Gd in Tissues of Interest in SFN (PNS and Footpads)

Among the organs of interest in the context of the study of the Small Fiber Neuropathy (SFN), the spinal cord is the matrix with the lowest Gd exposure (0.159–0.327 nmol/g*w depending on the GBCA), followed by the footpads (0.453–0.726 nmol/g*w), the sciatic nerve (1.293–1.869 nmol/g*w), and the spinal nodes showing the highest Gd exposure (1.770–2.590 nmol/g*w).

Gadopiclenol exhibits the lowest Gd exposure over the D1-M5 period. Gd washout in these structures is almost complete in 5 months (−91 to −99% vs W1 [no data at D1]). The overall exposure over the W1-M5 period in the PNS organs is +40% and +48% higher for gadoterate and gadobutrol respectively as compared to gadopiclenol (Fig. 11).

Gd Overall Exposure, a Summary

After gadopiclenol administration at the HED of 0.05 mmol/kg, the exposure vary between 0.101 nmol/g*w in the plasma to 296.1 nmol/g*w in the kidney cortex. The AUC in CNS is around 0.25 nmol/g*w, followed by the PNS (0.16–1.8 nmol/g*w), the skin (2.32 nmol/kg*w), the spleen, the bone marrow, and liver (around 7 nmol/g*w), then the bone epiphysis and diaphysis (19 nmol/g*w). The highest AUC is seen in the kidney, with 94–296 nmol/g*w.

The AUCs for the studied organs after administration of gadobutrol at the HED of 0.1 mmol/kg vary between 0.191 nmol/g*w for the plasma to 531.5 nmol/g*w in the kidney cortex. The AUC in CNS is around 0.45 nmol/g*w, followed by the PNS (0.3–2.6 nmol/g*w), the skin, the liver, the spleen, and bone marrow (7–12 nmol/g*w), then the bone epiphysis and diaphysis (18 nmol/g*w). The highest AUC by far is seen in the kidney, with 145–531 nmol/g*w.

The AUCs in the different tissues after administration of gadoterate at the HED of 0.1 mmol/kg vary between 0.163 nmol/g*w for the plasma to 435 nmol/g*w in the kidney cortex. The AUC in CNS is about 0.45 nmol/g*w, followed by the PNS (0.33–2.3 nmol/g*w), the skin (3.1 nmol/kg*w), the spleen, the liver, and bone structures (7.6–12.5 nmol/g*w). The highest AUC is seen in the kidney, with 117–435 nmol/g*w.

All GBCAs being administered at their respective HED, the overall cumulative exposure on the studied organs is 25% and 40% lower after gadopiclenol as compared to gadoterate or gadobutrol respectively (Fig. 11).

% Injected Dose

At each time post injection, the levels of Gd can also be expressed as the ratio of Gd amount in 1 g of tissue to the injected dose (%ID/g of organ). This gives an estimation of the residual Gd level in the tissues relative to the total amount of Gd injected (Table 2). In CNS, the rough estimate percentage of Gd to the injected dose accounts for no more than 1 × 10⁻⁴ %ID (≈ 1 millionth of the ID) after 1 day and 1 × 10⁻⁵ %ID (≈ 0.1 millionth of the ID) 5 months after injection in the whole brain.

TABLE 2.

Percentage of ID Within All the Organs and the Corresponding Order of Magnitude of the Gd Content Relative to the Injected Dose

			%ID/g (=10 −2 ID/g)	Organ Weight (*)	%ID/Organ (=10 −2 ID/Organ)
Brain	D1	Gadoterate	1.85 × 10−4	1.9 g	3.51 × 10−4
		Gadobutrol	1.73 × 10−4	2.0 g	3.48 × 10−4
		Gadopiclenol	1.70 × 10−4	1.7 g	2.82 × 10−4
		Order of magnitude	≈10 −4		≈10 −4
	M5	Gadoterate	3.28 × 10−6	2.3 g	7.51 × 10−6
		Gadobutrol	3.11 × 10−6	2.3 g	7.11 × 10−6
		Gadopiclenol	3.45 × 10−6	2.4 g	8.18 × 10−6
		Order of magnitude	≈10 −6		≈10 −5
Cortical kidney	D1	Gadoterate	2.72 × 10−1	1.06 g	3.14 × 10−1
		Gadobutrol	3.35 × 10−1	1.04 g	3.81 × 10−1
		Gadopiclenol	3.46 × 10−1	1.04 g	3.96 × 10−1
		Order of magnitude	≈10 −1		≈10 −1
	M5	Gadoterate	9.84 × 10−5	1.26 g	1.24 × 10−4
		Gadobutrol	1.80 × 10−4	1.24 g	2.22 × 10−4
		Gadopiclenol	2.13 × 10−4	1.30 g	2.78 × 10−4
		Order of magnitude	≈10 −4		≈10 −4
Medullar kidney	D1	Gadoterate	6.77 × 10−2	0.54 g	3.62 × 10−2
		Gadobutrol	7.94 × 10−2	0.48 g	3.81 × 10−2
		Gadopiclenol	9.02 × 10−2	0.50 g	4.60 × 10−2
		Order of magnitude	≈10 −1		≈10 −2
	M5	Gadoterate	1.84 × 10−4	0.56 g	1.04 × 10−4
		Gadobutrol	2.83 × 10−4	0.46 g	1.29 × 10−4
		Gadopiclenol	9.58 × 10−4	0.54 g	5.12 × 10−4
		Order of magnitude	≈10 −4		≈10 −4
Kidney tot	D1	Gadoterate	≈10 −3 for gadopiclenol 2.07 × 10−1	1.8 g	≈10 −3 / 10 −4 for gadopiclenol 3.76 × 10−1
		Gadobutrol	2.59 × 10−1	1.7 g	4.45 × 10−1
		Gadopiclenol	2.67 × 10−1	1.8 g	4.75 × 10−1
		Order of magnitude	≈10 −1		≈10 −1
	M5	Gadoterate	1.25 × 10−4	1.9 g	2.31 × 10−4
		Gadobutrol	2.08 × 10−4	2.0 g	4.20 × 10−4
		Gadopiclenol	4.29 × 10−4	2.0 g	8.69 × 10−4
		Order of magnitude	≈10 −4		≈10 −4 ≈10 −3 for gadopiclenol
Skin	D1	Gadoterate	4.04 × 10−3	~45 g at D1	1.82 × 10−1
		Gadobutrol	6.50 × 10−3		2.93 × 10−1
		Gadopiclenol	3.83 × 10−3		1.72 × 10−1
		Order of magnitude	≈10 −3		≈10 −1
	M5	Gadoterate	2.49 × 10−7	~65 g at M5	1.6 × 10−5
		Gadobutrol	< LLOQ		-
		Gadopiclenol	<LLOQ		-
		Order of magnitude	≈10 −7		≈10 −5 or less
Liver	D1	Gadoterate	8.14 × 10−3	9.03 g	7.35 × 10−2
		Gadobutrol	1.03 × 10−2	8.96 g	9.26 × 10−2
		Gadopiclenol	1.11 × 10−2	9.03 g	1.00 × 10−1
		Order of magnitude	≈10 −2		≈10 −1
	M5	Gadoterate	1.51 × 10−5	8.07 g	1.22 × 10−4
		Gadobutrol	1.80 × 10−5	9.24 g	1.66 × 10−4
		Gadopiclenol	3.19 × 10−5	9.19 g	2.93 × 10−4
		Order of magnitude	≈10 −5		≈10 −4
Spleen	D1	Gadoterate	8.25 × 10−3	0.76 g	6.31 × 10−3
		Gadobutrol	9.16 × 10−3	0.81 g	7.43 × 10−3
		Gadopiclenol	9.59 × 10−3	0.78 g	7.45 × 10−3
		Order of magnitude	≈10 −2		≈10 −2
	M5	Gadoterate	4.98 × 10−5	0.73 g	3.66 × 10−5
		Gadobutrol	8.88 × 10−5	0.87 g	7.75 × 10−5
		Gadopiclenol	1.02 × 10−4	0.82 g	8.40 × 10−5
		Order of magnitude	≈10 −4		≈10 −4
Femurs (epi + dia + marrow)	D1	Gadoterate	2.27 × 10−3	1.73 g	3.94 × 10−3
		Gadobutrol	3.03 × 10−3	1.78 g	5.40 × 10−3
		Gadopiclenol	4.92 × 10−3	1.73 g	8.53 × 10−3
		Order of magnitude	≈10 −3		≈10 −2
	M5	Gadoterate	3.67 × 10−4	2.22 g	8.16 × 10−4
		Gadobutrol	5.40 × 10−4	2.40 g	1.30 × 10−3
		Gadopiclenol	1.08 × 10−3	2.25 g	2.42 × 10−3
		Order of magnitude	≈10 –3 / 10 −4		≈10 −3
Bone matrix (based on [Gd] in mineral femur	D1	Gadoterate	2.11 × 10−3	~19 g at D1	~4 × 10−2
		Gadobutrol	2.89 × 10−3		~5 × 10−2
		Gadopiclenol	4.87 × 10−3		~9 × 10−2
		Order of magnitude	≈10 −3		≈10 −1
	M5	Gadoterate	3.76 × 10−4	~28 g at M5	~1.1 × 10−2
		Gadobutrol	5.56 × 10−4		~1.6 × 10−2
		Gadopiclenol	1.06 × 10−3		~3.0 × 10−2
		Order of magnitude	≈10 −3		≈10 −2

ID, injected dose.

In the 2 kidneys, the Gd found represents around 0.1%ID (≈ 1 thousandth of the ID) at D1, but the good washout of the Gd/GBCAs leads to around 1 × 10⁻⁴ %ID (≈ 1 millionth of the ID) to 1 × 10⁻³ %ID for gadopiclenol at M5, placing this organ at the second place of retention among the studied organs for gadopiclenol (order of magnitude similar to the liver and spleen for gadoterate and gadobutrol).

In the 2 complete femurs, the total Gd measured accounts for 1 × 10⁻² %ID at D1, and the poor washout explain that still 1 × 10⁻³ %ID is retained at M5. The total bone mass accounts for around 5–10% of a rat's bodyweight.^29,30 The female rats included in the present study weighted around 250 g at D1 and 370 g at M5, allowing us to estimate their bone mass at around 19 g at D1, and 28 g at M5. It can be calculated that around 0.1%ID is found in the skeleton 24 h after injection, and 1 × 10⁻² %ID (≈0.1 thousandth of the injected dose) is still accumulated at M5. This relatively high content of Gd, combined with the weight of the bone matrix in the bone leads the skeleton at the first place in terms of Gd retention.

The skin represents 15–20% of the bodyweight (Charles River's abacus source), which is around 45 g at D1 and 65 g at M5. It can then be estimated that for the tested macrocyclics, around 0.1 %ID is measured in the skin 24 hours post GBCA administration, but the major washout results in less than 1 × 10⁻⁵ %ID (≈0.1 millionth of the ID) 5 months later (only quantifiable for gadoterate, Gd concentrations below the LLOQ for gadobutrol and gadopiclenol), despite a large skin weight in the body.

In the liver and spleen, the Gd measured at D1 accounts respectively for 0.1%ID (≈1 thousandth of the injected dose) and 1 × 10⁻² %ID (≈0.1 thousandth of the ID). Five months later, the Gd accumulated represents around 1 × 10⁻⁴ %ID (≈1 millionth of the ID).

These results indicate that the final concentration levels are significantly lower than the initial injected dose.

DISCUSSION

Gd Use and Current Concerns

GBCAs are used in the daily routine for diagnostic and follow-up of patients by MRI. About 2 decades ago, a link was established between the NSF and the use of GBCAs in patients with severe renal impairment.¹ Studies have also highlighted that Gd can be retained in the healthy central nervous system (CNS), as evidenced by hypersignals in brain, and can also be found in various organs such as the skin, kidneys, and bones in patients with normal kidney function. These phenomena are particularly concerning linear GBCAs, which are characterized by a lower kinetic inertness, and therefore more likely to release free Gd into the body. Even more recently, preclinical studies have raised questions about a potential link between the small fiber neuropathy (SFN) and prior administrations of GBCAs, link not yet established. This disease, generally characterized by pain, burning sensations, and tingling, involves damage to the small fibers of the peripheral nervous system. These small fibers in the skin relay sensory information about pain and temperature. A first study was conducted on mice that received a single intravenous injection of either linear or macrocyclic GBCA at a clinical dose.⁵ One month after administration, a decrease in intraepidermal nerve fiber density was observed in all groups and terminal axonal swellings in linears. The effect was class-dependent, with the most significant changes induced by gadodiamide (linear, nonionic), followed by gadobenate (linear, ionic), and finally gadoterate, gadoteridol, and gadobutrol (macrocyclic). Another study, conducted on rats that received cumulated doses of 12 or 50 mmol/kg of gadodiamide, reported thermal and mechanical hyperalgesia, with a dose-related effect.³¹ Gadoterate was also included in the study, but no effect was reported for this macrocyclic GBCA. Similarly, Bilgin et al published a study describing that an administration of gadodiamide (1 mmol/kg), gadobutrol (0.5 mmol/kg), or gadobenate dimeglumine (0.5 mmol/kg) produced hyperalgesic effects, exacerbating pain in a migraine model in mice, when injected 30 minutes to 2 hours before the sensitivity tests.³² Other articles have reported hyperalgesia associated with the administration of linear GBCAs in mice and rats in various models^33–35 and/or histopathological alterations.^33,36 However, 2 studies in rats did not report such histopathological effects.^7,34 A recent small clinical study on 28 patients investigated whether the exposure to GBCAs to patients with idiopathic (unexplained) SFN was associated with Gd deposition and a decreased IENFD (intraepidermal nerve fiber density), compared to patients that did not receive any GBCA and to controls. They did not establish any causal effect between a previous injection of a GBCA and the pathological effects.⁶

Some authors described the concept of GDD (gadolinium deposition disease), a condition that occurs when Gd remains in the body's tissues even after the contrast agent has been excreted through the kidneys.³⁷ However, this concept is challenged as no pathological effect (except NSF) has been observed so far in human. Therefore, a more cautious position has recently been proposed by The American College of Radiology Committee on Drugs and Contrast Media. They suggested using the term SAGE (Symptoms Associated with Gadolinium Exposure) to describe symptoms reported after intravascular exposure to GBCAs.⁴ This term helps researchers and clinicians describe these symptoms without prematurely attributing them to a specific disease. In parallel, due to these concerns, there is an increasing caution within the radiologist community regarding the use of GBCAs, adhering to a careful benefit/risk balance. Radiologists now strive to minimize the use of contrast agent injections when the diagnosis can be made by other means or to use contrast agents that allow for a reduced dose.

Gadopiclenol, a New Generation of GBCA

Gadopiclenol (Elucirem/Vueway) is a recently approved macrocyclic and nonionic GBCA for human use, with at least 2-fold higher relaxivity thanks to the presence of 2 inner-sphere water molecules per gadolinium, and the highest kinetic inertness than the other marketed GBCA in acidic media.^9,10 Thanks to its high relaxivity, gadopiclenol is indicated by FDA and EMA at half Gd-dose (0.05 mmol Gd/kg bodyweight) compared to the other nonspecific marketed GBCAs. Two phase-3 clinical trials proved the diagnostic equivalence versus gadobutrol in CNS¹⁵ and body MRI.¹⁶

AUC and Global Exposure

The impact of this dose reduction on the overall Gd exposure to major body organs has not been documented in humans, as it would require longitudinal Gd quantification. This study aims to address this gap by estimating the cumulative amount of Gd present in various body organs within the first few months following injection, depending on the contrast agent used. To achieve this, rats were injected either with gadopiclenol at half the Gd dose or with 2 marketed products at full dose (gadoterate or gadobutrol), and then sacrificed at different time points to measure concentrations from the first hours (3.5 hours) to 5 months after injection. Based on these data, the area under the curve (AUC) estimating the overall Gd exposure was calculated after adjusting for the changes in concentration over time for each organ. The multiorgans overall exposure to Gd between the earliest and latest time points studied was estimated through the calculation of the area under the curve.

A Reduced Gd Exposure after Gadopiclenol

After gadopiclenol administration in rats at the HED, the overall Gd exposure between 1 day or 1 week after administration and 5 months was found to be inferior compared to the other tested macrocyclic GBCAs in the plasma, in the CNS (cerebellum, cortical brain, subcortical brain, and brain stem), in the PNS (spinal cord, spinal nodes, footpads, sciatic nerve), in bone marrow, in spleen, and in skin, liver, and kidney. In the femur, the Gd exposure was higher compared to the other macrocyclic GBCAs (or equivalent to the one in gadobutrol in the epiphysis).

The kidney is consistently the organ with the highest exposure to Gd. This is expected, as the kidney is the primary organ responsible for excreting GBCAs. Nearly every molecule of Gd that is eliminated from the body passes through the kidney to be excreted into the urine. However, a recent paper from our group demonstrated that the residual Gd found in the kidney after gadopiclenol administration remained under stable chelated forms.¹⁰

The second organ with the highest exposure to Gd is the mineral bone. Although Gd concentrations measured 1 day after injection are not the highest within all tissues, the washout from mineral bone structures is very limited for all GBCAs, from 1 week after administration. Bone has already been described as a deep compartment for Gd storage,³⁸ with ×4 to ×25 times higher long-term Gd retention observed in human sample of bone tissue after linear GBCAs compared to macrocyclics.³⁹ Moreover, speciation studies have shown that bone is the only studied matrix where retained Gd is likely to be dechelated or no longer in its original ligand, even for the macrocyclic GBCAs gadoterate, gadobutrol, and gadoteridol.⁴⁰ Even if no explanation is available to date, some hypotheses are under discussion: for example, the high affinity of Gd to hydroxyapatite crystals, which are rich in calcium and phosphate, combined with acidic conditions in areas of bone remodeling, might favor the dechelation of some circulating GBCAs, leading to long-term Gd storage. Femur diaphysis is the only tissue where gadopiclenol results in higher overall Gd exposure compared to the 2 macrocyclic contrast agents tested in this study (gadoterate and gadobutrol, ×1.8 and ×1.2, respectively). This is observed for femur epiphysis as well, but only as compared to gadoterate (×1.5), Gd exposure being identical as compared to gadobutrol. The structural differences between the diaphysis (compact bone) and epiphysis (spongy, cancellous bone), which result in lower hydroxyapatite content in the epiphysis, may influence the differences in long-term exposure. Additionally, the original chemical structure of gadopiclenol, such as its two-fold higher molecular weight, its pyridine moiety and its large number of hydroxyl groups,⁹ likely contributes to its slightly increased affinity for mineral bone, even when only half the usual dose of Gd is injected. Further investigations especially the speciation of Gd are needed to understand this exception for gadopiclenol in comparison to other tissues. Nevertheless, it is important to note that the residual Gd represents a poor fraction of the injected Gd (≈0.1 thousandth), and that to date, no safety concerns have been raised regarding Gd retention in bone, even for linear agents. Toxicological studies consisting in single and repeated high doses of gadopiclenol did not reveal any deleterious effect in the bone and bone marrow.²² A recent exhaustive review discusses the GBCAs/Gd retention in bone and interactions between Gd3+ and bone matrices, assessing current knowledge regarding the mechanism of retention and possible consequences.⁴¹

Following bone tissue, the Gd exposure is rather high for all GBCAs in the spleen, liver, and bone marrow. The liver and the spleen are indeed, after the kidney, the soft tissues with the highest Gd concentrations in the short term. The Gd washout is quite effective and complete in the liver and bone marrow, while it takes longer time to eliminate from the spleen. The spleen belongs to the reticuloendothelial system. It contains a lot of lymphatic cells, as lymphocytes, granulocytes, and macrophages, that filter the blood from pathogens, debris, damaged cells, and particles. Hence, it is plausible to consider that spleen captures some of the circulating GBCA. Indeed, DiGregorio and her team published several articles along those lines. In 2013, they published that GBCAs (gadodiamide, gadoteridol, and gadopentetate tested) were, in vitro, internalized in macrophages and fibroblasts endosomes and that the least stable GBCAs underwent dechelation through lysosomal digestion in macrophages.⁴² In 2018, they published data showing that the spleen was the first or second principal organ of Gd storage in mice in the medium term, regardless of the GBCA class: 3 weeks after injections of gadopentetate,⁴³ gadodiamide, or gadoteridol⁴⁴ and 3 months after injection of gadoteridol. Our data are consistent with theirs.

The skin is moderately exposed. Although the concentrations are not very high and the Gd washout is rather good, it is important to keep in mind that the skin covers a large area. Follow in terms of Gd exposure the organs from the PNS associated with the small fiber neuropathy (spinal node and sciatic nerve first, followed by the footpads), and CNS and spinal cord. The concentrations in the PNS at W1 are around 10 times higher than those in CNS organs, as previously reported in another SFN-related study.³¹

Skin is the organ of interest for historical record, through NSF. In NSF, it is most likely that dechelated Gd after injection of low stability GBCAs is responsible of fibrocyte recruitment and migration, inducing fibrosis. In NSF human skin samples (patients that received at least 1 administration of linear GBCA), Gd had been found in macrophages and fibrotic areas,⁴⁵ but also in vascular walls, in basement membranes of sweat glands and subcutaneous collagen.^46,47 However, the precise location of Gd retention in our model after gadoterate, gadobutrol, and gadopiclenol injection is unknown. Regarding the Gd speciation in the skin, Birka et al reported intact gadoteridol, a macrocyclic GBCA, 8 years after the administration to the patient.⁴⁸

Limitations

This study has some limitations. First, the Gd exposure in each organ is only an approximation of the true exposure because it is based on a limited number of sampling time points (at 3–4 time points, with some additional points for skin and plasma). The accuracy of the estimation is significantly influenced by the numbers of the initial time points, during which Gd concentrations fluctuate considerably due to the body's dynamic distribution following bolus injection. Subsequently, as the anticipated changes are relatively slow, the timing of sampling becomes less critical. In our study, the first time point studied is 1 day after injection. An earlier time point would have increase the accuracy to approximate more closely the Gd exposure. It is reasonable to think that it would also have reflected more the benefit of a half-Gd dose administration with gadopiclenol. Second, the exposure is expressed in terms of concentration, not mass balance. The concentration of Gd in nmol/g is an appropriate metric to link with potential biological effects, as it represents the quantity of extrinsic molecules observed by a gram of tissue. However, this does not reflect the overall quantity of Gd (total mass or moles), as precise values of organ weights are needed for that. A complete mass balance of Gd may be interesting to estimate by appropriate in vivo methods, such as radiolabeled ligand and/or radioactive gadolinium, as it was done for example with ¹⁵³Gd-EOB-DTPA or ¹⁵³Gd-DOTA, ¹⁵³Gd-DTPA, and ¹⁵³Gd-DTPA-BMA.^49,50 Third, this study has been done on healthy rats, which intrinsically limits the conclusions with regard to extrapolation to human. Anatomical differences between rats and humans include for example significant variations in organ size, which can affect the distribution of drugs and in terms of pharmacokinetics, drugs can be distributed, metabolized, and eliminated differently between the 2 species. However, rat models have been extensively used as animal models for long-term Gd retention investigations and have proven to be quite similar in terms of contrast agent ranking, especially comparing macrocyclics to linears GBCAs.⁵¹

In conclusion, this study confirms that the reduction of Gd dose of a macrocyclic contrast agent reduces the Gd exposure compared to the other marketed GBCAs at full dose. The mineral bone is the only organ with a greater exposure for gadopiclenol over the studied period. Under our experimental conditions, the overall measured 5-month Gd exposure following gadopiclenol injection at the HED is 25–40% lower than that observed after gadoterate and gadobutrol injections.

ORCID ID'S

Marlène Rasschaert https://orcid.org/0000-0001-6930-6858

Philippe Robert https://orcid.org/0000-0001-8744-7363