**Studying Side Effects of Tyrosine Kinase Inhibitors in a Juvenile Rat Model with Focus on Skeletal Remodeling Juvenile Rat Model with Focus on Skeletal Remodeling**

**Studying Side Effects of Tyrosine Kinase Inhibitors in a** 

DOI: 10.5772/intechopen.70006

Josephine T. Tauer, Bernadette A. S. Jäger, Anna Ulmer, Paula Geidel, Vera Girke, Lysann Kroschwald and Meinolf Suttorp Anna Ulmer, Paula Geidel, Vera Girke, Lysann Kroschwald, and Meinolf Suttorp Additional information is available at the end of the chapter

Josephine T. Tauer, Bernadette A. S. Jäger,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70006

#### **Abstract**

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The tyrosine kinase (TK) inhibitor (TKI) imatinib provides a highly effective treatment for chronic myeloid leukemia (CML) targeting at the causative oncogenic TK BCR-ABL1. However, imatinib exerts off-target effects by inhibiting other TKs that are involved, e.g., in bone metabolism. Clinically, CML patients on imatinib exhibit altered bone metabolism as a side effect, which translates into linear growth failure in pediatric patients. As TKI treatment might be necessary for the whole life, long-term side effects exerted on bone and other developing organs in children are of major concern and not yet studied systematically. Here, we describe a new juvenile rat model to face this challenge. The established model mimics perfectly long-term side effects of TKI exposure on the growing bone in a developmental stage-dependent fashion. Thus, longitudinal growth impairment observed clinically in children could be unequivocally modeled and confirmed. In a "bench-to-bedside" manner, we also demonstrate that this juvenile animal model predicts side effects of newer treatment strategies by second generation TKIs or modified treatment schedules (continuous vs. intermittent treatment) to minimize side effects. We conclude that the results generated by this juvenile animal model can be directly used in the clinic to optimize treatment algorithms in pediatric patients.

**Keywords:** juvenile, growth, bone, tyrosine kinase inhibitor, side effects, CML

#### **1. Introduction**

The introduction of tyrosine kinase inhibitors (TKIs) for targeted treatment of chronic myeloid leukemia (CML) marked a paradigm shift in the field of hemato-oncology [1, 2]. However, soon after CML became most successfully treated cancer—first in adults and thereafter in

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

children—it was learned that chronic exposure to TKIs impaired modeling of the osseous skeleton as an off-target effect [3]. This skeletal side effect resulted in impaired longitudinal growth in not outgrown minors [4, 5]. With regard to a potential lifelong necessity of TKI intake, children with CML differ from a typical patient with CML who is about 60 years old [6]. Thus, the rational of the research of TKIs' off-target effects is to generate a clear picture of early and late sequelae of long-term drug intake.

oncogene [10, 11]. This t(9; 22) translocation or Philadelphia chromosome (Ph+) is a characteristic cytogenetic abnormality seen in 95% of patients with CML and in 15–30% of adult patients with acute lymphoblastic leukemia (ALL) [12, 13]. The *BCR-ABL1* oncogene codes for two forms of fusion transcripts: p190BCR-ABL1 and p210BCR-ABL1, which are constitutively highly activated and subsequently dysregulate intracellular signaling by enhancing proliferative capability and resistance to apoptosis of hematopoietic stem or progenitor cells, leading to a

Studying Side Effects of Tyrosine Kinase Inhibitors in a Juvenile Rat Model with Focus on Skeletal...

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About 1–1.5/100,000 residents are diagnosed with CML every year with an age peak between 50 and 60 years [14], representing around 20% of all cases of leukemia in adulthood [15]. Concerning pediatric patients, the frequency of diagnosis is about 0.05–0.40/100,000 residents per year within the age of 0–18 years [16]. Thus, CML represents one of the rarest leukemic disorders in childhood and adolescent age, accounting for only 2–3% of all children suffering from leukemia [16]. In terms of morphological characteristics, childhood CML is not different from adult CML. However, it is a matter of an ongoing debate whether and to what extend molecular differences exist between CML diagnosed at childhood or older age [6]. For example, pediatric CML shows a breakpoint distribution in the *BCR* gene more similar to

Still, as the *BCR-ABL1* oncogene is the single molecular aberration causing the development of CML, specific TKIs like imatinib (Gleevec®, Novartis) have been developed to inhibit the BCR-ABL1 TK [19]. By achieving hematological and cytogenetic response in over 90% of the patients after a few months of imatinib treatment, imatinib has been very effective in inhibit-

**Figure 1.** Survival probabilities by year of diagnosis (1980–2013) of pediatric patients with CML in Germany [18].

massive increase in myeloid cell numbers.

ing progression of CML (**Figure 1**) [1, 20–23].

adult Ph+ ALL [17].

On this background, the essential objective of this chapter is the description of a juvenile (still growing) rat model that allows a chronic administration of TKIs via the drinking water in order (i) to mimic osseous changes observed in humans, (ii) to further characterize and investigate the causative pathophysiologically mechanisms resulting in impaired bone growth, (iii) to test approaches in growing animals for ameliorating the off-target effect resulting in growth impairment, and (iv) to check further organs beside bone for long-term TKI toxicity.

In this chapter, sections describe i) the highly effective role that TKIs play in standardized attempts to operationally cure CML in adults as well as in children, ii) elucidate the role of the established juvenile male Wistar rat model to investigate with ease the skeletal changes at all developmental stages, and iii) focus on the administration of TKI via the drinking water over many weeks as an adequate and convenient way resulting in the achievement of therapeutic drug blood levels. TKI-induced changes in long bones, as well as vertebrae, can be investigated with dedicated small imaging devices while blood levels of bone turnover markers, growth hormone, and vitamin D metabolites can be followed at different stages of development. The results of these investigations as well as the derived hypothesis on the pathophysiological cascade, specifically how TKIs impair longitudinal bone growth, are in excellent agreement with clinical observations. In addition, the juvenile animal model is of value to monitor other long-term TKI side effects on the heart and fertility to generate an overall picture on all possible side effects.
