**3.4. Tyrosine kinase inhibitors**

Tyrosine kinase inhibitors (TKIs) have emerged as a new class of molecular targeted anticancer agents with proven efficacy in several types of carcinoma and in some hematologic cancers. At the moment, about 150 kinase-targeted drugs are in clinical development and many more are in various stages of preclinical development [95].

The registered kinase inhibitors are small molecules, that, with sporadic exceptions, exert their pharmacological action at the ATP-site of a wide variety of tyrosine kinases critical for tumor cell survival and proliferation and angiogenesis. TKIs compete with ATP for binding to the catalytic domain that act by preventing ATP binding to these targets with different selectivity, potency and pharmacokinetic properties. However, despite their inherent selectivity, the available TKIs showed a variable grade of affinity for the different tyrosine kinases, but none specific for a single kinase. In addition, interference is frequently extended to off-target intracellular processes of normal cells. As a direct consequence, TKIs may cause a number of infrequent or even new toxicities. TKI-induced endocrine side effects mainly include thyroid dysfunction and disease, but also may be responsible for gonadal and adrenal dyfunction, bone and glucose altered metabolism, impairment in linear growth and fetal development [96].

Two types of thyroid dysfunctions have been observed with TKIs [97]. The first is worsening of hypothyroidism in patients under satisfactory treatment with thyroid hormone replacement. This effect was initially noted in patients under treatment with imatinib, but also other TKIs were found responsible for increase in LT4 supplementation in hypothyroid patients [98-102]. The second type of thyroid disturbance is primary hypothyroidism in patients with previously normal thyroid function. Almost all TKIs are responsible, at a variable extent, of primary hypothyroidism, with the exception of gefitinb and erlotinib.

The mechanisms of TKI-induced thyroid dysfunction are unclear. An increase in the requirement of replacement therapy with LT4 during certain TKIs has been suggested to be dependent on possible interference of the drug at nondeiodination clearance of LT4 [98]. Several drugs (e.g. phenobarbital, phenytoin, carbamazepine, rifampicin, and nicardipine) can increase thyroid hormone clearance through the induction of hepatic microsomal enzymes, including mixed function oxygenases and uridine diphosphate– glucuronosyltransferases [103]. These drugs can cause hypothyroidism in patients who undergo LT4 [11,104-106]. For example, imatinib is a potent competitive inhibitor of several mixed function oxygenases (CYP2C9, CYP2D6, and CYP3A4/5), and the induction of uridine diphosphate–glucuronosyl transferases has been hypothesized to be a possible mechanism of interference of imatinib on levothyroxine metabolism [98]. Alternatively, an interference of sunitinib with thyroid hormone action at the pituitary level has been suggested [122].

TKI-induced hypothyroidism (*de novo* primary hypothyroidism) may have several explanations. In some cases thyrotoxicosis may precede the development of hypothyroidism, suggesting a thyroiditis-induced thyrotoxicosis [107,108,123]. Other possible mechanisms include direct toxic effects on thyrocytes, namely the reduced synthesis of thyroid hormones related to inhibition of thyroid peroxidase activity [109], impaired iodine uptake [110], the drug induced regression of the gland vascular bed with significant capillary alteration and reduction in density [111]. However, a role for iodine uptake in TKI-hypothyroidism was not confirmed by *in vitro* studies [112] and antiperoxidase effect seems unable to provide an explanation for cases with initial destructive thyrotoxicosis, or with thyroid atrophy.

Induction of Hashimoto thyroiditis has also been proposed [113], although Hashimoto thyroiditis is unlikely to be the main mechanism because of the low prevalence of anti-TPO antibodies.

The most likely explanation is that the thyroid dysfunction may be related to the effects of these agents on tyrosine kinase receptors involved in vascular function, such as vascular endothelial growth factor (VEGF) receptor (VEGFR). This could cause a reduction in thyroid blood flow to this extremely vascular gland. If the blood flow decreases rapidly, an ischemic thyroiditis could result, leading to a transient period of thyrotoxicosis. If the decreased blood flow develops more slowly, gradual thyroid destruction may occur with resulting hypothyroidism [114]. Supporting evidence for this theory include findings that thyroid cells express VEGF and VEGFR mRNA, and preclinical studies in mouse models have shown glandular capillary regression with TKI exposure [115]. In humans, case reports demonstrated reduced thyroid volume and reduced vascularity by Doppler ultrasound [116,117], with rapid increase in the size of the thyroid with cessation of sunitinib. This reduced thyroid volume secondary to reduced blood flow, may also explain the impaired radioactive iodine uptake *in vivo*, but not *in vitro* [118]. However, the role of VEGF in thyroid signaling is uncertain. Unlike treatment with antiangiogenic TKIs, bevacizumab is not associated with altered thyroid homeostasis [119]. In addition, *in vitro* experiments showed that VEGF reduces TSH induced iodine uptake by thyroid cells, and inhibition of VEGF restores iodine uptake [120]. Other factors, such as platelet derived growth factor α and c-KIT, contribute to maintaining thyroid homeostasis, but so far no data on their role in this toxicity have been published.

#### *3.4.1. Imatinib*

318 Thyroid Hormone

concomitant hypophysitis or other IRAEs.

**3.4. Tyrosine kinase inhibitors** 

fetal development [96].

and erlotinib.

be assessed before initiating treatment and before each cycle.

many more are in various stages of preclinical development [95].

hydrocortisone, if required. None of the patients affected by anti-CTLA4-IT showed

Patients who need to receive anti–CTLA-4 mAbs should be carefully informed on the importance of observing and early reporting signs or symptoms potentially related to IRAEs, and that these symptoms may occur weeks to months after starting treatment. In these patients, TSH, free-T4, serum electrolytes, serum glucose, and blood cell counts should

Tyrosine kinase inhibitors (TKIs) have emerged as a new class of molecular targeted anticancer agents with proven efficacy in several types of carcinoma and in some hematologic cancers. At the moment, about 150 kinase-targeted drugs are in clinical development and

The registered kinase inhibitors are small molecules, that, with sporadic exceptions, exert their pharmacological action at the ATP-site of a wide variety of tyrosine kinases critical for tumor cell survival and proliferation and angiogenesis. TKIs compete with ATP for binding to the catalytic domain that act by preventing ATP binding to these targets with different selectivity, potency and pharmacokinetic properties. However, despite their inherent selectivity, the available TKIs showed a variable grade of affinity for the different tyrosine kinases, but none specific for a single kinase. In addition, interference is frequently extended to off-target intracellular processes of normal cells. As a direct consequence, TKIs may cause a number of infrequent or even new toxicities. TKI-induced endocrine side effects mainly include thyroid dysfunction and disease, but also may be responsible for gonadal and adrenal dyfunction, bone and glucose altered metabolism, impairment in linear growth and

Two types of thyroid dysfunctions have been observed with TKIs [97]. The first is worsening of hypothyroidism in patients under satisfactory treatment with thyroid hormone replacement. This effect was initially noted in patients under treatment with imatinib, but also other TKIs were found responsible for increase in LT4 supplementation in hypothyroid patients [98-102]. The second type of thyroid disturbance is primary hypothyroidism in patients with previously normal thyroid function. Almost all TKIs are responsible, at a variable extent, of primary hypothyroidism, with the exception of gefitinb

The mechanisms of TKI-induced thyroid dysfunction are unclear. An increase in the requirement of replacement therapy with LT4 during certain TKIs has been suggested to be dependent on possible interference of the drug at nondeiodination clearance of LT4 [98]. Several drugs (e.g. phenobarbital, phenytoin, carbamazepine, rifampicin, and nicardipine) can increase thyroid hormone clearance through the induction of hepatic microsomal enzymes, including mixed function oxygenases and uridine diphosphate– glucuronosyltransferases [103]. These drugs can cause hypothyroidism in patients who

Imatinib inhibits the kinase activity of the tyrosine kinases of breakpoint cluster region proto-oncogene ABL1 (BCR-ABL fusion protein), c-Kit and platelet derived growth factor receptor (PDGFR) /β. It is currently approved for the treatment of chronic myeloid leukemia, gastrointestinal stromal tumors (GIST) and dermatofibrosarcoma protuberans. The influence of daily 400–800 mg imatinib on LT4 was reported in a cohort of 11 patients (10 with medullary thyroid carcinoma and 1 with GIST) [98]. Among the patients with medullary thyroid carcinoma, eight underwent thyroidectomy and received LT4 and three had thyroid carcinoma *in situ.* Thyroid function was evaluated before, during and 2 weeks after therapy with imatinib or LT4. Symptoms of hypothyroidism occurred in all patients who had undergone thyroidectomy, but not in those with intact thyroid. Patients who had undergone thyroidectomy had markedly elevated TSH levels, and required an increase of LT4 during imatinib dosing. The effect was reversible after discontinuation of treatment, suggesting that imatinib might be the causative agent.

Thyroid Function Abnormalities in Patients Receiving Anticancer Agents 321

15 patients with hypothyroidism, 6 (40%) had at least one TSH value below 0.5 mU/L before developing the condition, which suggests a thyroiditis-induced thyrotoxicosis. The risk of hypothyroidism increased with the duration of sunitinib therapy. Subclinical or overt hypothyroidism was observed in 4 out of 22 patients [18%] who received sunitinib for 9 months, and in 5 of 17 patients (29%) who received sunitinib for longer than 12 months. In patients treated for longer than 96 weeks, 90% developed increased levels of TSH. The mean time to onset of hypothyroidism was 50 weeks. Among the patients with TSH concentrations greater than 10 mU/L, none had spontaneous biochemical resolution. During the titration of LT4, serum TSH values remained elevated for a median of 17 weeks (range 4–117 weeks). The TSH concentrations returned to normal in all patients who received conventional doses of LT4. Interestingly, in 2 patients with hypothyroidism and normal baseline TFTs, ultrasonography revealed atrophic thyroid tissue, which suggests destructive thyroiditis. This clinical trial was the first to report the prevalence of sunitinib related

Rini *et al.* [111] described thyroid abnormalities in a retrospective study of 66 patients with metastatic RCC treated with sunitinib. In all, 30 patients were pretreated with cytokinebased therapy (6 of them were treated with bevacizumab), and 30 patients were treatmentnaive. All patients received the standard sunitinib dose of 50 mg daily for 4 weeks, followed by 2 weeks off therapy. TFT assessment, including free thyroxine index, was initiated in 29 patients (and subsequently in another 37 patients) as a routine laboratory assessment at baseline and on day 28 of every even numbered cycle. Out of the 66 patients, 56 (85%) had one or more TFT abnormality. These abnormalities were consistent with hypothyroidism in all patients and primarily included the elevation of TSH, decreased levels of T3 and, less commonly, decreases in T4 and/or of the free thyroxine index. TFT abnormalities were detected early (the median time of detection was at cycle 2). Among patients with abnormal TFTs, signs and symptoms related to hypothyroidism were found in 47 patients (84%). These symptoms included fatigue, cold intolerance, anorexia, periorbital edema, fluid retention, and alterations in skin or hair. LT4 therapy was given at the discretion of the physician, on the basis of the degree of biochemical abnormality and/or clinical symptoms. A resolution of biochemical abnormalities occurred in all 17 patients treated with LT4, and an improvement of symptoms was recorded in 9 patients. Anti-TG antibodies were measured in 44 patients and were abnormal in 13 (30%). No association was observed between the presence of anti-TG antibodies and the incidence or severity of TFT

Feldman *et al.* [124] reported that hypothyroidism was found in 14 (18%) out of the 80 patients enrolled in a prospective clinical trial that investigated the efficacy of sunitinib in metastatic RCC. Serum TSH levels were obtained only from symptomatic patients and ranged from 6.0 to 146.4 mU/L (normal range 0.35–5.5 mU/L). Hypothyroidism was detected after a median time of 10 months of therapy (range 1–26 months), being fatigue the predominant symptom. The Authors highlighted that the lower incidence of hypothyroidism reported might depend on the fact that TFT assessment was performed on

hypothyroidism [123].

abnormalities [111].

symptomatic patients only [124].

In another study on 68 patients with intact thyroid gland who received imatinib for chronic myeloid leukemia, no case of drug-induced alterations in thyroid laboratory parameters was observed. These data sustain the hypothesis that imatinib-induced thyroid dysfunction is limited to athyreotic patients and are not the consequence of a direct action of the drug on thyroid gland [121], but more probably of an interference of the drug in the nondeiodination clearance of LT4 through the induction of hepatic microsomal enzymes [98].
