Clinical impact of retinoids in redifferentiation therapy of advanced thyroid cancer: final results of a pilot study

Differentiated thyroid cancer is a malignant tumour that has a fairly good prognosis, with patients surviving for many years. Multimodal therapy with surgery, radioiodine therapy and TSH suppressive medication is of proven efficacy. However, loss of differentiation is observed in up to one-third of patients with differentiated thyroid cancer, paralleled by an increase in tumour grading and loss of thyroid-specific functions (thyrotropin receptor, iodine accumulation). Such tumours may no longer be amenable to standard treatment protocols, including TSH suppression and radioiodide therapy. Retinoic acids have been shown to exert re-differentiating effects on thyrocytes in various experimental studies and case reports, and it was on this basis that this pilot study was initiated. Patients with advanced thyroid cancer and without the therapeutic options of operation or radioiodide therapy were treated with 13- cis-retinoic acid at a dosage of 1.5 mg/kg body weight daily over 5 weeks. Parameters for assessment of the therapeutic effect were serum thyroglobulin (TG) levels, radioiodine uptake, and tumour size prior to and after retinoid treatment. Fifty patients were evaluated for response, classified as reduction in tumour size and TG levels, stable disease or disease progression. Thirteen patients showed a clear increase in radioiodine uptake, and eight a mild increase. TG levels were unchanged or decreased in 20 patients. Tumour size was assessable in 37 patients; tumour regression was observed in six, and there was no change in 22. In total, a response was seen in 19 patients (38%). Response to retinoid therapy did not always correlate with increased radioiodine uptake, so other direct antiproliferative effects have to be assumed. The encouraging results of the study and the low rate of side-effects with good tolerability of retinoids warrant further studies with altered inclusion criteria and employment of other redifferentiating drugs or combinations of agents.