Gene Therapy for Thyroid Cancer: Current Status and Future Prospects

Christine Spitzweg; John C. Morris

Disclosures

Thyroid. 2004;14(6) 

In This Article

NIS Gene Therapy

Cloning of the NIS gene and its extensive characterization has not only revolutionized our understanding of the physiology and pathophysiology of thyroidal iodide accumulation, but also provided us with a powerful new diagnostic and therapeutic gene.[44,45] As an intrinsic plasma membrane glycoprotein, NIS mediates the active transport of iodide at the basolateral membrane of thyroid follicular cells. NIS cotransports one iodide ion against its electrochemical gradient together with two sodium ions along their electrochemical gradient (Fig. 2).[46]

Schematic model of the protein structure of the human sodium iodide symporter (NIS) protein.

Functional NIS expression in the thyroid gland is responsible for thyroidal accumulation of iodide, an essential constituent of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). The unique property of thyroid follicular cells to trap and concentrate iodide because of expression of NIS allows imaging as well as highly effective therapy of differentiated thyroid carcinomas and their metastases by administration of radioiodine, thereby improving the prognosis of thyroid cancer patients significantly.[47] Differentiated thyroid carcinomas are usually treated by total or near-total thyroidectomy, followed by 131I ablation of the thyroid remnant and occult microscopic carcinomas. Subsequent postablative 131I total body scanning can diagnose local and metastatic residual and recurrent disease. Therapy with 131I has been successfully used for more than 40 years in the treatment of differentiated thyroid cancer. Recurrence rates are significantly higher in patients treated with surgery and thyrotropin suppression by thyroxine alone compared to those who also receive radioiodine treatment.[47] The efficacy of radioiodine therapy is reflected in the low mortality of patients suffering from metastatic thyroid cancer who are treated with 131I (3%) as compared to those who are not (12%). Even young patients with diffuse pulmonary metastases at initial presentation can be successfully treated by 131I, achieving a 10-year survival of more than 80%.[47] Thyroidal NIS expression therefore opens the door to effective cancer therapy with remarkably low incidence of serious adverse affects.

Cloning of the NIS gene[44,45] has paved the way for the development of a novel cytoreductive gene therapy strategy for the treatment of thyroidal and extrathyroidal malignancies based on NIS gene transfer followed by radioiodine therapy. Targeted expression of functional NIS in cancer cells would enable these cells to concentrate iodide from plasma, and would, therefore, offer the possibility of radioiodine therapy. Application of the NIS gene as novel therapeutic gene therefore extends the diagnostic and therapeutic use of 131I and the extensive experience with radioiodine in the management of differentiated thyroid cancer to the treatment of nonthyroidal cancer and dedifferentiated, anaplastic, and medullary thyroid cancer. NIS further represents a therapeutic gene that is associated with a bystander effect, because not only NIS-transduced cancer cells, but also surrounding non-transduced cells are destroyed by the crossfire effect of the β-emitter 131I, that is characterized by a path length of 0.2-2.4 mm.[48]

Several investigators have explored the efficacy of NIS gene transfer into nonthyroidal tumor cells for induction of radioiodine accumulation allowing radioiodine therapy of extrathyroidal malignancies. Using various gene delivery techniques, including electroporation, liposomes, adenoviral and retroviral vectors, radioiodine accumulation was induced in vitro and in vivo in a variety of cancer cell lines (glioma and neuroblastoma cells, melanoma, cervix, breast, lung, liver, colon and ovarian carcinoma cells, myeloma cells, pancreatic neuroendocrine tumor cells) by NIS gene delivery.[49,50,51,52,53,54,55,56,57] In addition, prostate cancer (LNCaP) cells were shown to be selectively killed by accumulated 131I after induction of tissue-specific iodide uptake activity by prostate-specific antigen (PSA) promoter-directed NIS expression in vitro.[58,59] Iodide accumulation was confirmed in vivo in LNCaP cell xenografts in athymic nude mice and was high enough to allow a therapeutic effect of 131I in vivo. A single therapeutic 131I dose of 3 mCi was administered and shown to elicit a dramatic therapeutic response in NIS-transfected LNCaP cell xenografts with an average volume reduction of more than 90%.[59] As a next crucial step toward therapeutic application of NIS gene delivery followed by radioiodine therapy in patients with prostate cancer in a clinical setting, a replication-deficient human adenovirus carrying the human NIS gene linked to the CMV promoter (Ad-5CMV-NIS) was used to perform in vivo NIS gene transfer into LNCaP cell tumors. After intraperitoneal injection of a single therapeutic dose of 3 mCi 131I 4 days after adenovirus-mediated intratumoral NIS gene delivery, LNCaP xenografts showed a clear therapeutic response with an average volume reduction of more than 80%.[60] These studies clearly showed for the first time that NIS gene delivery into nonthyroidal nonorganifying tumor cells is capable of inducing accumulation of therapeutically effective radioiodine doses, and might therefore represent an effective and potentially curative therapy for extrathyroidal tumors.

While in differentiated thyroid cancer functional NIS expression allows effective therapy with radioiodine, patients with poorly differentiated thyroid cancer with low TSH-stimulated NIS expression levels or patients with medullary thyroid cancer do not benefit from 131I therapy because of insufficient or absent 131I accumulating activity. In these patients NIS gene transfer could be used to restore or induce 131I accumulation, thereby establishing effective 131I therapy. Early studies in malignantly transformed rat thyroid cells (FRTL-Tc) without iodide transport activity showed that transfection with rat NIS cDNA using electroporation is able to restore radioiodine accumulation in vitro and in vivo. However, the effective half life of 131I in NIS-transfected FRTL-Tc xenografts in rats was only 6 hours and did not allow a therapeutic effect of 131I (1 mCi).[61] More recently, stable transfection of a NIS-defective follicular thyroid carcinoma cell line (FTC-133) with the NIS gene was able to reestablish iodide accumulation activity in vitro and in vivo.[62] In the same NIS-transfected follicular thyroid carcinoma cell line, thyroid ablation and low-iodide diet were able to increase the biologic half-life of accumulated radioiodine in vivo from 3.8 hours to 26.3 hours leading to postponed xenotransplant development in nude mice after administration of a therapeutic dose of 2 mCi 131I.[63] Petrich et al.[64] transiently transfected a variety of papillary, follicular, and anaplastic thyroid carcinoma cell lines (B-CPAP, K1, WRO, 8505C, FTC-133) with the NIS gene, thereby inducing perchlorate-sensitive accumulation of 125I. These studies show that NIS gene delivery into thyroid cancer cells is capable of restoring 131I accumulation, and might therefore represent an effective therapy for dedifferentiated and anaplastic thyroid tumors that lack iodide-accumulating capacity.

Lee et al.[65] used a recombinant adenovirus to transduce a panel of thyroid carcinoma cell lines (ARO, FRO, NPA) with the NIS gene. They demonstrated significant iodide accumulation associated with rapid iodide efflux in vitro, which the authors explained by low expression levels of Tg, TSH-receptor, and thyroid peroxidase (TPO) genes involved in iodide organification.[65]

The radiation dose responsible for a possible therapeutic effect of trapped 131I is determined by the rate of uptake of iodide and its retention time in the tumor cell, which is affected by rate of iodide efflux, iodide recirculation, and iodide binding to cellular proteins or lipids, a process termed iodide organification. In the thyroid gland trapped iodide is organified by TPO-catalyzed oxidation and incorporation into tyrosyl residues along the Tg backbone. Enhancing the retention time of 131I in the thyroid gland, iodide organification is generally believed to be a crucial prerequisite for 131I therapy, although no clear data exist to prove this hypothesis. In fact, our recent studies in LNCaP cells, that do not organify iodide after PSA promoter-mediated NIS gene transfer, clearly demonstrate that iodide organification is not required for a therapeutic effect of 131I in tumor cells.[58,59,60] Several investigators have asked if coupling of NIS gene transfer with the delivery of the TPO gene in nonorganifying tissues is capable of inducing iodide organification thereby enhancing the iodide retention time and achieved radiation dose in the target tissue. In a recent study, Boland et al.[66] showed that coinfection of human cervix carcinoma cells with two different adenoviral vectors encoding the rNIS gene and the hTPO gene, respectively, does not increase iodide retention time in the tumor cells, although enzymatically active TPO was produced and a significant increase in iodide organification could be observed. In contrast to these findings, Huang et al.[67] demonstrated that cotransfection of the NIS and TPO genes in non-small-cell lung cancer cell lines was capable of decreasing iodide efflux suggesting that the degree of organification-mediated iodide retention might be cell-type-specific. Whether TPO-mediated organification can indeed be used as a therapeutic strategy to enhance the efficacy of NIS-based radioiodide concentrator gene therapy remains to be confirmed in further studies.

Recently, the high energy α-emitter 211astatine (211At) and the potent β-emitter 188rhenium (188Re), have been introduced as alternative radionuclides to 131I for treatment of thyroidal and nonthyroidal tumors after NIS gene transfer.[68,69,70,71] It has been known from cell culture and animal experiments that 211At and 188Re accumulate in the thyroid similar to iodide and pertechnetate, suggesting that they are transported by NIS. Petrich et al.[70] demonstrated sodium-dependent and perchloratesensitive accumulation of 211At in papillary and anaplastic thyroid carcinoma cell lines (B-CPAP, K1, 8505C) after stable transfection with the NIS gene, resulting in a significant increase of the tumor absorbed dose from 3.5 Gy/MBqtumor for 131I to 50.3 Gy/MBqtumor for 211At. These data suggest that application of 211At may enhance the differentiated follicular cell-derived thyroid carcinomas and therapeutic efficacy of NIS-based gene therapy, in particular medullary thyroid cancer after NIS gene transfer (Fig. 3). in thyroid tumors with low iodide retention time.[70]

Concept of the sodium iodide symporter (NIS) gene as novel diagnostic as well as therapeutic gene.

Finally, recently, in medullary thyroid cancer, a therapeutic effect of radioiodine was demonstrated following induction of tissue-specific iodide uptake activity by calcitonin promoter-directed NIS gene transfer in vitro.[72]

Taken together, these studies demonstrate the potential of NIS as a novel therapeutic gene allowing 131I therapy of dedifferentiated follicular cell-derived thyroid carcinomas and medullary thyroid cancer after NIS gene transfer (Fig. 3).

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