Aluminium from adjuvanted subcutaneous allergen immunotherapeutics in rats is mainly detected in bone
Authors: Karin Weisser, Thomas Göen, Jennifer D. Oduro, Gaby Wangorsch, Kay‐Martin O. Hanschmann, Brigitte Keller‐Stanislawski
Journal: Allergy (2019)

To the Editor, Aluminium (Al) hydroxide (AH) is frequently used as adjuvant in subcutaneous allergen immunotherapy (SCIT) in Europe. With regard to the cumulative Al amount administered per year, SCIT constitutes an important source of Al in humans, and its safety is constantly questioned in the public. Toxicokinetics of Al after SC injection of those poorly soluble Al particles is unknown. We conducted a study in rats receiving a single SC injection (1 mL) of either plain AH adjuvant (pAH: Alhydrogel®, 1.25 mg Al/mL; n = 6 rats), each of two marketed AH-adjuvanted SCIT products at highest marketed strength with high Al content but different in adjuvant manufacture (P1: Alhydrogel®-adjuvanted mixed grass pollen allergen, 1 mg Al/mL; P2: in situ co-precipitated AH-adjuvanted mixed grass pollen allergen, 1.13 mg Al/mL; n = 6 rats each), or vehicle (saline; n = 3 rats). Al in plasma (during 80 days) and tissues (at day 80; whole humerus bone and right brain hemisphere) was determined by atomic absorption spectrometric (AAS) procedures (full Method section in Appendix S1). Al plasma time courses after treatment did not exhibit profiles distinctive from that of the vehicle group (Figure 1); however, for P2, but not for P1 and pAH, a significant increase in Al plasma AUC(0-80 d) vs vehicle (mean [SD]:1431 [314] vs 909 [48] µg/L*d) was found. Al concentration in bone was significantly increased in all groups (Figure 2; Table S2), again strongest after P2 (mean [SD]: 1.28 [0.26], 0.64 [0.12], 0.72 [0.10] and 0.40 [0.11] µg/g wet weight (ww) for P2, P1, pAH and vehicle, respectively). However, concentrations were far below the levels of toxicological concern (>10-15 µg/g in both rats and humans1, 2). A more visible increase in bone rather than plasma exposure is not surprising as in contrast to fast renal Al plasma clearance, elimination of Al from bone is very slow and skeleton is the major storage compartment for Al in the body of both rats and humans.3 No significant difference in brain Al concentration between any treatment and vehicle was observed (means: 0.07-0.29 vs 0.08 µg/g ww; Figure 2; full data see Table S2), which is consistent with observations of very small fractions of dose (<0.01%) retaining in brain.3 Since we measured the Al amount in a whole hemisphere, clusters due to focal accumulation as well as any undissolved Al particles potentially transported into the brain4 are fully captured. We conclude from our results that contribution of such particulate Al amounts in brain, if any, is negligible. The findings in plasma and bone were corroborated by determinations at the injection site: a high Al release (65.9% of injected dose) was found for P2 in contrast to small dose fractions for plain Alhydrogel® and Alhydrogel®-adjuvanted product P1 (1.7 and 11.2%, resp.; Figure S3 and Table S2). The highest absolute Al amount released after 80 days was 659.1 µg for P2, being about fivefold compared with that for P1 (126.6 µg). Frequency of persistent nodules at day 80 was higher in the groups receiving pAH (6/6 rats) and P1 (5/6) compared with P2 (3/6), showing a plausible inverse relation to the Al release from the injection site. A positive linear relationship was found between Al amount released from the injection site until day 80 and Al plasma AUC(0-80 d) (y = 0.73x + 927; r = .79) or bone Al concentration (y = 0.00079x + 0.63; r = .70), respectively (Figure S4). These correlations confirm that Al release was an indirect measure of the systemically available amount. However, we cannot exclude some overestimation of this amount as a fraction of insoluble AH might have been transported to the draining lymph node but not yet entered the system. To our knowledge, this is the first time that Al from marketed SCIT products has been systemically detected. Increase in Al levels was mainly visible in bone, less in plasma, but not in brain. Furthermore, our data indicate that Al-adjuvanted SCIT products do not behave uniformly: we observed a remarkable difference in the degree of systemic Al availability at day 80 between two marketed AH-adjuvanted SCIT products, suggesting a higher rate of Al release from the co-precipitated product. This is most probably due to the physicochemical differences (ie chemical composition and amorphous state) of co-precipitated AH adjuvants compared with preformed commercial AH adjuvants such as Alhydrogel®.5 However, once absorption is completed, dose-normalized cumulative extent of bone Al increase is expected to be comparable between products. Crude linear extrapolation of our injection site release data over time (including literature data6) suggests that complete absorption from Alhydrogel®-adjuvanted allergen preparations takes at least 350 days, whereas absorption from co-precipitated products might be completed much earlier (after ca. 120 days). Our results cannot one-to-one be translated to humans. In relation to body weight, the doses applied to our rats (350 g) were 170 times higher compared with a 60 kg human adult. Considering an allometric scaling factor of 6.2 usually applied for dose conversion on mg/kg basis between rats and humans in pharmacology,7 this ratio is still 27. Thus, increase in bone Al concentration in a human adult after absorption of a SCIT Al dose of 659 µg (estimated Al release after P2 on day 80) is assumed 1/27 of that observed in our rats. Based on this calculation, we expect that after a single SCIT dose in humans, Al levels in bone, and even more valid in plasma and brain, will be indistinguishable from baseline levels. However, assuming full accumulation during a 3-year perennial SCIT applying 36 maintenance doses each containing up to 1250 µg Al in adults a cumulative bone Al increase of 1-2 µg/g wet weight is roughly extrapolated. Compared with an upper limit of normal bone Al content in healthy humans of <10 µg/g dry weight,1, 8 this increase is considered substantial but without clinical relevance. For more precise predictions of Al exposure according to various treatment schedules, particularly in children, a physiology-based toxicokinetic (PBTK) model is desirable.9 For its establishment, the results of this study are considered essential. Author Jennifer D. Oduro declares that she is employee at preclinics GmbH, a contract research organization that has received payment for the conductance of the animal study. All other authors declare that they have no conflict of interest. This work was supported by the German Ministry of Health (ZMVI1-2515-FSB-772). Open access funding enabled and organized by projektDEAL. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.