Skeletal Effects of the Tyrosine Kinase Inhibitors Imatinib, Dasatinib, and Bosutinib in Young Rats
Authors: Tauer Josephine Tabea , Hofbauer Lorenz C. , Erben Reinhold G. , Suttorp Meinolf 
Published: October 2019

Abstract
Abstract 4429

Background:
In patients with chronic myeloid leukemia (CML) tyrosine kinase inhibitors (TKIs) such as imatinib (IMA), dasatinib (DASA), and bosutinib (BOSU) are used to selectively inhibit the oncogenic BCR-ABL tyrosine kinase. However, these TKIs also exhibit off-target effects on other kinases (e.g. PDGFR, c-FMS) involved in the regulation of bone metabolism. In adult patients with CML, disturbances in bone structure and metabolism are observed during IMA therapy, while in pediatric patients longitudinal growth retardation represents a major concern. Here, we compared the skeletal effects of chronically administered IMA, DASA, and BOSU in young, growing rats.

Methods:
From age 4 weeks to age 14 weeks, groups (n=10 each) of male Wistar rats were chronically exposed via the drinking water to concentrations representing i) standard dose and ii) two-fold dose of the standard dose of IMA (i: 1 mM and ii: 2 mM), DASA (i: 50 μM and ii: 100 μM), or BOSU (i: 50 μM and ii: 100 μM). Additional groups received the high doses of the TKIs tested in an intermittent fashion over 3 consecutive days per week. Controls were left untreated. Three to four rats of each group were sacrificed after 2 weeks (prepubertal), 4 weeks (pubertal), or 10 weeks (postpubertal) of exposure, respectively. Biochemical bone markers were analyzed in serum. Bone length of femora and tibiae was determined with calipers. Trabecular (trab) and cortical (cort) bone mineral density (BMD) and cortical thickness were assessed by pQCT. Bone strength of femora was measured by a 3-point bending test.

Results:
Continuous exposure to IMA and DASA led to dose dependent reductions in femoral and tibial length. No such effect was observed under BOSU exposure. Intermitted exposure of all 3 TKIs tested reduced effects on femoral and tibial length. IMA and DASA lowered femoral and tibial trab BMD in prepubertal rats. At pubertal age, rats receiving high dose IMA showed reduced femoral and tibial trab BMD compared to controls, while no such effect was observed for DASA and BOSU. Postpubertally, femoral and tibial trab BMD did not differ from controls in all groups. Moreover, TKI exposure did not affect femoral or tibial cortical BMD and cortical thickness at any dose and age tested. The bone resorption marker tartrate resistant acidic phosphatase (TRACP) was reduced in rats receiving continuously high dose of IMA at all time points tested, and also in postpubertal rats receiving 50 μM and 100 μM of BOSU. The bone formation marker osteocalcin was reduced in all groups and at all time points under IMA exposure, whereas no such effect was observed for DASA and BOSU. Femoral mechanical properties did not differ from controls pre- and pubertally for all 3 TKIs tested. However, high dose IMA reduced femoral breaking strength in postpubertal rats (Fig.1). DASA or BOSU did not cause changes in biomechanical bone strength.

Conclusion:
The juvenile rat model is an appropriate model to examine side effects of long-term exposure of TKIs on growing bones. Compared with IMA and DASA, BOSU seems to exert a less negative effect on the bone parameters tested. Given the limited pediatric experience with TKI, regular monitoring of skeletal side effects under long-term TKI treatment is warranted. Moreover, skeletal status and fracture rates should be systematically analyzed in patients under IMA treatment.

Acknowledgment:
Supported by grant DFG SU122-3/1 to MS, DFG HO 1875/10-1 to LCH, and FWF I 734 to RGE.

Disclosures:
No relevant conflicts of interest to declare.