Table 1: In vitro/in vivo experiments and clinical studies on the association between GLP-1 analogs and C-cell pathology.

Authors [reference]Studied drugsResearch materials or study subjectsMain outcomes investigatedMain findings

                 Cellular models

Crespel et al. [32]Glucagon, GLP-1 (7–36), and exendin (9–39)Rat CA-77 C-cell linecAMP production and calcitonin secretionGLP-1 (7–36) and glucagon dose dependently stimulated cAMP production and calcitonin secretion. Exendin (9–39) abolished a further increase in cAMP formation at glucagon concentration over 10 nM and partially suppressed glucagon-induced calcitonin secretion.

Lamari et al. [31]GLP-1 (7–37), and exendin (9–39)Rat CA-77 C-cell linecAMP production, calcitonin mRNA levels, and calcitonin secretionGLP-1 (7–37) increased cAMP formation in a dose-dependent manner. Exedin (9–39), an antagonist of GLP-1 receptor, blunted the stimulation of cAMP production induced by GLP-1 (7–37). Gene expression and peptide secretion of calcitonin were increased after incubation of CA-77 cells with GLP-1 (7–37).

Knudsen et al. [11]Liraglutide, exenatide, and GLP-1 (7–37)Human TT C-cell line, rat MTC 6–23 C-cell line, and rat CA-77 C-cell lineGLP-1 receptor mRNA and protein expression; calcitonin release after GLP-1 receptor agonistsNative GLP-1, liraglutide, and exenatide all stimulate calcitonin gene expression and calcitonin secretion via the GLP-1 receptor in a dose-dependent manner in rat C cells. The human TT cells express few GLP-1 receptors compared with rat MTC 6–23 and CA-77 and show a lack of functional response to GLP-1 and GLP-1 receptor agonists.

                 Animal experiments

(I) Rodents
 Knudsen et al. [11]Liraglutide versus vehicle controlSprague Dawley rats aged 6-7 weeks and CD-1 mice at the age of 5–10 weeksPlasma calcitonin and pathological examination to thyroid gland sections after dosing with liraglutideCalcitonin levels increase with time and dose with 104-week repeated dosing of liraglutide. The incidences of both C-cell hyperplasia and C-cell tumor formation at 104 weeks were increased in a dose-dependent manner and reached statistical significance.

 Madsen et al. [33]Liraglutide, exenatide, and vehicle controlCD-1 wild-type mice aged 5-6 weeks and GLP-1-receptor knockout mice at the age of 4-5 weeksPlasma calcitonin, pathological examination to thyroid tissue sections, and immunohistochemical staining for phosphoproteins after 13-week treatment with liraglutide or exenatideGLP-1 agonists cause calcitonin release and C-cell hyperplasia in wild-type mice via a GLP-1-receptor-dependent mechanism. GLP-1 activates the mammalian target of rapamycin (mTOR) pathway by stimulating the production of cAMP. Activation of mTOR in turn results in downstream phosphorylation of ribosome S6. Liraglutide-induced C-cell hyperplasia in mice is not associated with RET activation.

(II) Primates
 Knudsen et al. [11]Liraglutide versus vehicle controlCynomolgus monkeys aged 1-2 yearsPlasma calcitonin and pathological analysis to thyroid gland sections after dosing with liraglutideNo increase in plasma calcitonin was seen in cynomolgus monkeys receiving a single dose of liraglutide or during 87-week daily dose. There was also no change in the thyroid gland sections, relative C-cell fraction of the thyroid gland, and proliferative index in the C cells after liraglutide for 52 weeks.

                 Human studies

Gier et al. [10]Human thyroid glandsExpression of GLP-1 receptors in tissue samples with C-cell abnormalities, papillary thyroid cancer, and normal thyroidGLP-1 receptor immunoreactivities were detected in 33%, >90%, and 18% of patients with normal C cells, C-cell pathologies, and PTC lesions, respectively.

Hegedüs et al. [34]Liraglutide versus active comparators and placeboNine clinical trials of 20–104-week durationGeometric mean levels of serum calcitonin and outlier analysisThere was no significant difference in mean calcitonin levels between liraglutide and control groups. The proportions of subjects with calcitonin levels shifting to a higher category or above a clinically relevant cut-off value of 20 pg/mL were low and did not differ between treatment groups.

Elashoff et al. [36]Exenatide versus rosiglitazoneAdverse effect reporting systemOverall thyroid cancerOdds ratio for thyroid cancer was 4.73;