**3.3. Anti-CTLA4 monoclonal antibodies**

Ipilimumab and tremelimumab are monoclonal antibodies directed against cytotoxic Tlymphocyte-associated antigen 4 (CTLA-4) (anti–CTLA-4 mAbs), a receptor expressed on Tcells that exerts a suppressive effect on the immune response after T-cell/antigen-presenting cell interaction [80]. Blocking the receptor, an increased T-cell activation and antitumor effects are obtained. Ipilimumab is approved for the treatment of unresectable or metastatic melanoma.

These agents have been associated with several immune-related adverse events (IRAEs), most frequently enterocolitis, hepatitis, cutaneous reactions [81-83]. The spectrum of autoimmune endocrine adverse events experienced by patients treated with anti–CTLA-4 mAbs includes hypopituitarism, primary thyroid disease, and sporadically primary adrenal insufficiency [84]. The prevalence of autoimmune hypophysitis varies among different studies (0%–17%) [85], being 3-5% in larger studies. Similarly to classical autoimmune hypophysitis, secondary hypothyroidism has been reported in patients who develop hypophysitis induced by anti-CTLA-4 monoclonal antibodies.

316 Thyroid Hormone

[69].

melanoma.

immune-mediated destruction of tumor cells. They also inhibit proliferation and induce

Thalidomide and lenalidomide are approved for the treatment of multiple myeloma. Lenalidomide has also been approved for 5q myelodysplastic syndrome. Both agents are under evaluation for the treatment of several solid tumors, including thyroid cancer [65-67]

Hypothyroidism has been associated to treatment with these drugs with varying grades and frequency [68-71]. In a recent study on patients affected by multiple myeloma and treated with thalidomide [69], subclinical hypothyroidism was reported in 20% of participants, and 7% showed overt hypothyroidism, mostly occurring 1–6 months after initiating treatment

Lenalidomide is more potent and showed a more favorable toxicity profile compared to thalidomide [72,73]. Hypothyroidism due to lenalidomide has been reported in 5%–10% of patients [74,75]. Thyroid abnormalities were found in 10 out of 170 patients who received lenalidomide for various hematological cancers. After a median of 5 months of therapy the patients reported both hypothyroidism and thyrotoxicosis. However, many of them had

Many mechanisms have been suggested for the hypothyroidism induced by these drugs [69], including inhibition of thyroid hormone secretion [77] or a reduction of iodine uptake into follicular cells [78]. Most probably, since thalidomide and lenalidomide exert an antiangiogenic activity, compromise the blood flow to the thyroid may explain thyroid toxicity [69]. In some patients, TSH suppression has been documented before the development of hypothyroidism, suggesting ischemic thyroiditis [69]. Alternatively, a thyrotoxicosis triggered by an immune-mediated destructive thyroiditis may be hypothesized. This condition may be induced by deregulation of cytokine levels or through direct effects on T-lymphocytes [69]. A direct toxic effect on thyroid cells is also possible, but this has not been evaluated. TSH measurement before treatment and then every 2–3 months

Ipilimumab and tremelimumab are monoclonal antibodies directed against cytotoxic Tlymphocyte-associated antigen 4 (CTLA-4) (anti–CTLA-4 mAbs), a receptor expressed on Tcells that exerts a suppressive effect on the immune response after T-cell/antigen-presenting cell interaction [80]. Blocking the receptor, an increased T-cell activation and antitumor effects are obtained. Ipilimumab is approved for the treatment of unresectable or metastatic

These agents have been associated with several immune-related adverse events (IRAEs), most frequently enterocolitis, hepatitis, cutaneous reactions [81-83]. The spectrum of autoimmune endocrine adverse events experienced by patients treated with anti–CTLA-4 mAbs includes hypopituitarism, primary thyroid disease, and sporadically primary adrenal

apoptosis of tumor cells and exert antiangiogenic activity [63,64].

and for a range of autoimmune diseases [68].

been exposed to prior radiation or thalidomide [76].

during treatment is recommended [1,79].

**3.3. Anti-CTLA4 monoclonal antibodies** 

Direct damage to the thyroid induced by these agents presents two clinical common forms: hyperthyroidism in Graves' disease and thyroid destruction with hypothyroidism in Hashimoto's thyroiditis. Since these conditions are classically included in autoimmune thyroid diseases phenotype and previous studies suggested that *CTLA-4* is a candidate gene conferring susceptibility to thyroid autoimmunity [86], an autoimmune pathogenesis has also been suggested for thyropathies induced by anti-CTLA4-mAbs (anti-CTLA4-IT). The incidence of these conditions varies from 0 to 4% among the different trials. In two studies [87,88], tremelimumab (15 mg/Kg body weight) was associated with thyreopathy (hyper- /hypothyroidism, autoimmune-thyroiditis, or Graves' disease) in 4% of cases. In studies on ipilimumab, the reported incidence of anti-CTLA4-IT was apparently lower, namely 0-2%, being mild hypothyroidism the most frequent thyroid side-effect. Recently, Hodi et al. in a large phase III trial, reported for the first time an improvement in overall survival obtained by ipilimumab in pretreated patients affected by metastatic melanoma [89]. The treatment was associated with thyroid disorders or abnormal thyroid function tests in approximately 2% of patients. In a randomized phase II study on a cohort of 115 patients affected by metastatic melanoma, evaluating the potential protective effect of budesonide on IRAEs induced by ipilimumab, hypothyroidism was diagnosed in 5.3% of patients who received ipilimumab with placebo (3 out of 57 patients; with severe hypothyroidism in one of them, 1.8%), compared with no cases in the group of patients treated with budesonide as well. These data suggest a potential protective effect of budesonide in terms of reduced incidence of ipilimimab-related thyroiditis, but this hypothesis needs to be confirmed in specifically designed clinical trials [90].

In a phase I study [91] evaluating the combination of ipilimumab (10 mg/Kg every 3 weeks for 4 cycles, than every 3 months) with bevacizumab (7.5 mg/kg - Cohort 1; 15 mg/kg cohort 2 every 3 weeks) in a group of 21 patients affected by unresectable stage III or stage IV melanoma, thyroiditis was diagnosed in 4 (19%) patients. No cases of endocrine-IRAEs were reported in a trial on 36 patients who received both ipilimumab (0.1-3 mg/kg; 24 patients received the higher dose) and interleukin-2 (720,000 IU/kg every 8 hours) which is not infrequently associated with autoimmune thyroiditis [92].

The onset of anti-CTLA4-IT appears rather earlier than other IRAEs, occurring after 2-4 infusions. In most cases the anti-CTLA4-ITs have a subclinical course or may be transient. Alternatively, this condition may be characterized either by hypothyroidism with increased serum TSH concentrations, normal free-T4, and presence of anti-TPO antibodies, less frequently anti-TG antibodies or evolve in permanent hypothyroidism, requiring thyroid hormone supplementation [93].

Sporadic cases of Graves' ophthalmopathy associated with ipilimumab therapy in euthyroid patients have been reported [82,94]. In these cases the effective treatment was to administer a high dose of glucocorticoids in the acute phase, rapidly tapered down and continue with hydrocortisone, if required. None of the patients affected by anti-CTLA4-IT showed concomitant hypophysitis or other IRAEs.

Thyroid Function Abnormalities in Patients Receiving Anticancer Agents 319

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

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

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

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

thyrotoxicosis, or with thyroid atrophy.

toxicity have been published.

*3.4.1. Imatinib* 

antibodies.

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 be assessed before initiating treatment and before each cycle.
