Drugs that suppress TSH or cause central hypothyroidism

Many different drugs affect thyroid function. Most of these drugs act at the level of the thyroid in patients with normal thyroid function, or at the level of thyroid hormone absorption or metabolism in patients requiring exogenous levothyroxine. A small subset of medications including glucocorticoids, dopamine agonists, somatostatin analogues and rexinoids affect thyroid function through suppression of TSH in the thyrotrope or hypothalamus. Fortunately, most of these medications do not cause clinically evident central hypothyroidism. A newer class of nuclear hormone receptors agonists, called rexinoids, cause clinically significant central hypothyroidism in most patients and dopamine agonists may exacerbate 'hypothyroidism' in patients with non-thyroidal illness. In this review, we explore mechanisms governing TSH suppression of these drugs and the clinical relevance of these effects.

Bexarotene induces dyslipidemia by increased very low-density lipoprotein production and cholesteryl ester transfer protein-mediated reduction of high-density lipoprotein

A common dose-limiting side effect of treatment with the retinoid X receptor agonist bexarotene is dyslipidemia. We evaluated the effects of bexarotene on plasma lipid metabolism in patients with metastatic differentiated thyroid carcinoma and investigated the underlying mechanism(s) in apolipoprotein (APO) E*3-Leiden mice without (E3L) and with human cholesteryl ester transfer protein (CETP; E3L.CETP). To this end, 10 patients with metastatic differentiated thyroid carcinoma were treated with bexarotene (300 mg/d) for 6 wk. Bexarotene increased plasma triglyceride (TG; +150%), primarily associated with very low-density lipoprotein (VLDL), and raised plasma total cholesterol (+50%). However, whereas bexarotene increased VLDL-cholesterol (C) and low-density lipoprotein (LDL)-C (+63%), it decreased high-density lipoprotein (HDL)-C (-30%) and tended to decrease apoAI (-18%) concomitant with an increase in endogenous CETP activity (+44%). To evaluate the cause of the bexarotene-induced hypertriglyceridemia and the role of CETP in the bexarotene-induced shift in cholesterol distribution, E3L and E3L.CETP mice were treated with bexarotene through dietary supplementation [0.03% (wt/wt)]. Bexarotene increased VLDL-associated TG in both E3L (+47%) and E3L.CETP (+29%) mice by increasing VLDL-TG production (+68%). Bexarotene did not affect the total cholesterol levels or distribution in E3L mice but increased VLDL-C (+11%) and decreased HDL-C (-56%) as well as apoAI (-31%) in E3L.CETP mice, concomitant with increased endogenous CETP activity (+41%). This increased CETP activity by bexarotene-treatment is likely due to the increase in VLDL-TG, a CETP substrate that drives CETP activity. In conclusion, bexarotene causes combined dyslipidemia as reflected by increased TG, VLDL-C, and LDL-C and decreased HDL-C, which is the result of an increased VLDL-TG production that causes an increase of the endogenous CETP activity.

Rexinoid-induced expression of IGFBP-6 requires RARbeta-dependent permissive cooperation of retinoid receptors and AP-1

The synthetic rexinoid bexarotene (Targretin, LGD1069) inhibits the formation of both estrogen receptor-negative and estrogen receptor-positive breast cancer in preclinical models and controls the expression of growth-regulatory biomarkers, such as IGFBP-6 (insulin-like growth factor-binding protein 6), RARbeta, or cyclin D1. In this study, we identified a classical retinoic acid-responsive element in the first intron in the IGFBP-6 gene adjacent to a consensus AP-1 binding site, both elements essential for rexinoid-induced expression of IGFBP-6. In chromatin binding experiments, bexarotene increased the occupancy of the identified enhancer element by RXRalpha, RARbeta, cJun, cFos, and p300. In normal mammary epithelial cells and T47D breast cancer cells, small interfering RNA-mediated knockdown of all RXR isoforms or RARbeta, but not RARalpha or RARgamma alone, blocked the induction of IGFBP-6 by bexarotene. Simultaneous knockdown of RARalpha and RARgamma abrogated both the induction of RARbeta and the up-regulation and secretion of IGFBP-6. The suppression of either RARbeta or cJun by small interfering RNA blocked the recruitment of RXRalpha and cJun to the enhancer. These results demonstrate a novel cooperative interaction between retinoid receptors and AP-1 orchestrated by RARbeta and highlight a novel mechanism by which RARbeta can mediate the cancer-preventive effects of rexinoids.

Molecular biology and targeted therapy of cutaneous T-cell lymphomas

Cutaneous T-cell lymphoma (CTCL) is an extra-nodal non-Hodgkin lymphoma of mature T cells. These tumor cells home to and persist in the skin, producing a broad spectrum of clinical entities. Recent results of basic research on tumor biology and tumor immunology as well as molecular genetics of cutaneous T-cell lymphoma have fostered the development of new therapeutic approaches. Several clinical trials testing these targeted therapies have shown encouraging results. This article provides an overview of recent research developments and therapeutic strategies for cutaneous T-cell lymphoma.

CD8+ cutaneous T-cell lymphoma successfully treated with bexarotene: a case report and review of the literature

CD8+ cutaneous T-cell lymphoma (CTCL) is a relatively rare subset of the non-Hodgkins lymphomas. Bexarotene has been FDA-approved for the treatment of CTCL, but previous studies have been conducted on CD4+ CTL and there have been no reports about its use in CD8+ CTCL. Herein, we report on a patient whose CD8+ CTCL completely responded to treatment with bexarotene.

[Autologous hematopoietic stem cell transplantation followed by oral bexarotene in a patient with advanced mycosis fungoides]

We describe the case of a 17-year-old patient with rapidly progressing and aggressive mycosis fungoides, with multiple cutaneous tumors and large cell transformation. She was initially treated with 3 cycles of high-dose chemotherapy with mega-CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) without response, leading to the decision to undertake autologous hematopoietic stem cell transplantation. Partial remission of the disease was achieved with this treatment and subsequent introduction of oral bexarotene led to complete remission, which has been maintained for more than 3 years with good tolerance of oral therapy. We discuss the advantages and disadvantages of autologous hematopoietic stem cell transplantation and the use of oral bexarotene.

Retinoic acid receptor gamma activates receptor tyrosine kinase Tie1 gene transcription through transcription factor GATA4 in F9 stem cells

OBJECTIVE

The retinoic acid receptors (RARs) alpha, beta2, and gamma regulate specific subsets of target genes during all-trans retinoic acid (RA) induced differentiation of F9 teratocarcinoma stem cells. The Tie1 gene exhibited reduced expression in RA-treated F9 RARgamma-/- cells as compared to wild-type (WT) by microarray analysis. Our goal was to analyze the Tie1 gene, which encodes a surface receptor tyrosine kinase expressed in the hematovascular system.

MATERIALS AND METHODS

We assessed Tie1, Tie2, Flk1, Runx1, Peg/Mest2, and angiopoietin-1 and 2 mRNA levels and Tie1 promoter activity.

RESULTS

We showed that RARgamma, but not RARalpha or RARbeta2, is required for Tie1 promoter activation by RA. Treatment with a RARgamma selective agonist plus a retinoid X receptor agonist (LGD1069) increased Tie1 mRNA levels by 11- +/- 2.5-fold 48 hours after RA addition in F9 WT, but not in F9 RARgamma-/- cells, by quantitative reverse transcription polymerase chain reaction. Multiple putative GATA elements were identified in the Tie1 proximal promoter. RA increased GATA4 transcripts by 12- +/- 1-fold in F9 WT at 48 hours, but not in F9 RARgamma-/- cells. In addition, transfection of a GATA4 expression vector increased Tie1 promoter/luciferase activity in both RA-treated F9 WT and RARgamma-/- cells. Tie1 promoter deletion analyses indicated that a region of the promoter that possessed multiple GATA sites mediated the RA-associated Tie1 transcriptional increase.

CONCLUSIONS

Our results indicate that GATA4 plays a role in the RA/RARgamma-associated transcriptional activation of the Tie1 promoter. An understanding of RAR specificity in RA signaling should result in insights into hematopoietic stem cell signaling and potentially in improved therapies for several human diseases.

Radioiodine therapy after pretreatment with bexarotene for metastases of differentiated thyroid carcinoma

OBJECTIVE

To evaluate the effects of pretreatment with the retinoid X receptor (RXR) agonist bexarotene on the efficacy of radioiodine therapy for metastases of differentiated thyroid carcinoma (DTC) with limited uptake of radioiodine (I-131).

DESIGN

An open prospective intervention study.

METHODS

Eight patients with metastases of DTC, with insufficient uptake of I-131 who showed increased uptake of radioiodine after previous treatment with bexarotene were treated with radioiodine (7400 MBq), preceded by 6 weeks of treatment with bexarotene 300 mg/day. Outcome parameters were serum thyroglobulin (Tg) levels and dimensions of metastases at computed tomography (CT), measured before and 6 months after therapy. Tissues of primary tumours were stained with antibodies against retinoic acid receptor (RAR) and RXR subtypes.

RESULTS

Bexarotene pretreatment induced radioiodine uptake in metastases in all eight patients, although uptake was only discernable at single photon emission computed tomography (SPECT) and had incomplete matching with the metastases visualized by CT scanning. Six months after radioiodine therapy, six patients had progressive disease (defined as a > 10% increase in serum Tg and/or a > 25% increase in tumour dimensions), whereas two patients had stable disease. No relationship was observed between retinoid receptor staining pattern at immunohistochemistry and the outcome of therapy.

CONCLUSIONS

Bexarotene therapy did not result in restoration of susceptibility to radioiodine therapy.

Modulation by bexarotene of mRNA expression of genes in mouse lung tumors

Bexarotene has demonstrated chemopreventive and therapeutic efficacy towards mouse lung tumors. Using specimens from our published study that demonstrated the efficacy of bexarotene, we report herein its ability to modulate mRNA expression of genes in both lung and lung tumors. Strain A/J mice were administered vinyl carbamate to induce lung tumors. This was followed by 200 mg/kg body weight of bexarotene administered by oral gavage during Wks 4-25 or 23-25. The mice were sacrificed at Wk 25. The expression of 26 genes was decreased in lung tumors, whereas only two genes, Apolipoprotein D and CYP26b, had their mRNA expression increased by bexarotene. Genes with increased mRNA expression in untreated lung tumors include: epiregulin and kininogen-1 (increased by more than 40-fold) and Caspase-3, Cyclin D1, DNA methyltransferase 3a (Dnmt-3a), E-prostanoid 3 receptor (EP3), c-myc, surfactant protein-C, and survivin (increased by 1.7- to 3.6-fold). Bexarotene decreased the mRNA expression of Caspase-3, Dnmt-3a, EP3, and survivin, as well as the expression of the Cyclin E1, estrogen receptor-alpha, and iNOS genes. Bexarotene had a greater effect in decreasing the expression of Caspase-3, Cyclin E1, Dnmt-3a, EP3, iNOS, and survivin, when administered to mice with established tumors than when administered to mice while tumors were emerging. In summary, bexarotene modulated mRNA expression of genes in mouse lung tumors, being more effective in established tumors than in emerging tumors, suggesting that modulation of expression could be useful as a biomarker for the therapeutic and chemopreventive activity of the drug, especially in established tumors.

Cutaneous T cell lymphoma: translating immunobiology into therapeutic opportunities

Cutaneous T cell lymphoma (CTCL) has always served as a proving ground where conceptual advances in immunology can be tested and the results translated into clinical practice. From the earliest studies that used sheep red blood cells to identify the malignant cell as a T lymphocyte to molecular demonstration of the clonalilty of the disease, basic science techniques have provided sign posts that allow us to understand the clinical features seen in the patients. We continue to apply this paradigm to develop new insights into the role of the immune system in CTCL with the goal of using this knowledge to enhance the therapeutic options available to the patient. This article will review the studies that have led to our current understanding of the immunobiology of CTCL and the new therapeutic approaches that are being tested in this disease.

Folliculotropic mycosis fungoides responding to bexarotene gel

Folliculotropic mycosis fungoides (FMF) is an uncommon and potentially aggressive form of cutaneous T cell lymphoma (CTCL). Phototherapy, radiotherapy, and systemic chemotherapy are the most commonly employed treatment options, but may have limited success and common adverse reactions. Bexarotene gel is a topical retinoid X receptor (RXR) agonist with activity on the follicular unit that has not been previously reported in the management of FME The case of a 73-year-old male with FMF that responded to bexarotene gel is presented.

Safety and efficacy of low-dose bexarotene and PUVA in the treatment of patients with mycosis fungoides

BACKGROUND

The new rexinoid bexarotene is a retinoid X receptor antagonist and immune response modifier. Although combinations of oral bexarotene and psoralen plus UVA (PUVA) have been tried in patients with all stages of mycosis fungoides (MF), the dosage of bexarotene used in these combination regimens has been variable.

OBJECTIVE

To assess the efficacy and safety of low-dose oral bexarotene and PUVA in patients with relapsed or treatment-refractory MF following monotherapy with multiple agents including PUVA, narrow-band UVB, interferon-alpha, oral bexarotene, and topical corticosteroids.

METHOD

Combination therapy with PUVA three times weekly and low-dose oral bexarotene (150 or 300 mg/day, depending on physicians' preference) was administered to 14 patients, seven men and seven women (median age 49.5 years, range 30-75 years), with relapsed or refractory MF stages I-III. All responders received maintenance treatment at the same bexarotene dose that induced remission until progression or unacceptable toxicity.

RESULTS

Low-dose oral bexarotene combined with PUVA was associated with an overall response rate (complete response or partial response) in 67% of the nine patients with refractory MF who completed the treatment course. Of these nine patients, four had a complete response, two had a partial response, one had stable disease, and two had progressive disease. Five patients withdrew because of hyperlipidemia. Oral bexarotene was continued as maintenance therapy in three of the four complete responders (one refused); two of these patients relapsed 2-10 months after PUVA discontinuation. Patients with partial response or stable disease received the combination for 3-5 months and were switched to another treatment regimen because of lack of further response. Therapy was fairly well tolerated.

CONCLUSION

In a select population of patients who had not responded to at least one monotherapy for early-stage MF, a combination of low-dose oral bexarotene and PUVA was successful in achieving a satisfactory overall response rate in 67% of patients who completed the treatment course and was fairly well tolerated. Limitations of the study include the small number of patients evaluated, its retrospective nature, and the fact that patients were commenced on different bexarotene starting doses (150 or 300 mg/day), depending on physicians' preference.

Maintenance therapy in cutaneous T-cell lymphoma: Who, when, what?

The aim of current therapy for cutaneous T-cell lymphoma (CTCL) is to induce clinically meaningful remission, provide symptom relief, improve patient quality of life (QoL) and prolong disease-free and overall survival. A key research question is whether such remissions or minimal disease status can be maintained in the long term. There have been few formal studies of maintenance therapy in CTCL. Some skin-directed therapies such as total-skin electron-beam therapy and high-dose psoralen plus ultraviolet A may not be considered suitable, because of the risk of long-term cumulative toxicities. Other therapies such as nitrogen mustard, interferon (IFN)-alpha and bexarotene have demonstrated positive effects in prolonging remissions in small numbers of patients. Large longitudinal studies are required to investigate the efficacy of maintenance treatments in CTCL and their impact on patients' QoL and overall survival. Of the systemic therapies currently approved for the treatment of CTCL, bexarotene and IFN-alpha are obvious candidates for testing, because they can be self-administered by the patient and provide good long-term tolerability.