Bexarotene-induced hypothyroidism and dyslipidemia; a nation-wide study.

Central hypothyroidism and dyslipidemia are well-known adverse events (AEs) of bexarotene therapy. Although hypothyroidism is known to cause dyslipidemia, no study has examined the association between hypothyroidism and dyslipidemia in patients undergoing bexarotene therapy. The aim of this study is to examine this association. A retrospective observational study was performed among 294 patients who initiated bexarotene therapy in Japan (nation-wide postmarketing complete surveillance). Jonckheere-Terpstra (one sided) test was performed to evaluate the effect of the bexarotene dose on lipid metabolisms, and regression analyses were performed to evaluate associations of bexarotene dose, free thyroxine (FT4), body mass index (BMI), and lipid metabolisms. Most patients developed hypothyroidism. Two-third of patients showed FT4 values below the lower limit at 1 week. Triglycerides (TG) increased in a bexarotene dose-dependent manner, and grade ≥3 AEs on hypertriglyceridemia was observed in 39% of the patients. Additionally, one-third of grade ≥3 AEs on hypertriglyceridemia occurred within 1 week. The delta_FT4 (difference in FT4 from baseline) negatively correlated with TG increase at 1 week (p = 0.012) but not with low density lipoprotein cholesterol (LDL-C) increase at any week. Bexarotene-induced hypothyroidism is almost inevitable and occurred quickly. Bexarotene-induced hypertriglyceridemia showed positive bexarotene dose dependency and negative delta_FT4 dependency. Prophylactic and appropriate thyroid hormone compensation therapy and starting bexarotene at low doses with subsequent titration while managing dyslipidemia may have a beneficial effect for the successful continuation of bexarotene therapy without severe endocrine and metabolic AEs.

Successful prednisolone or calcimimetic treatment of acquired hypocalciuric hypercalcemia caused by biased allosteric CaSR autoantibodies.

Biased agonism is a frontier field in GPCR research. Acquired hypocalciuric hypercalcemia (AHH) is a rare disease caused by calcium-sensing receptor (CaSR) autoantibodies, to date, showing either simple blocking or biased properties (i.e., stimulatory or blocking effects on different downstream signaling pathways). This emphasizes the importance of the Gi/o (pertussis toxin-sensitive G proteins, whose ßγ subunits activate multiple signals, including ERK1/2) in regulating parathyroid hormone secretion. We here describe 3 patients with symptomatic AHH who shared characteristics with the 2 cases we previously reported as follows: (a) elderly (74-87 years at diagnosis), (b) male, (c) unexpectedly showed no other autoimmune diseases, (d) showed spontaneously fluctuating Ca levels from approximately normal to near fatally high ranges, (e) acute exacerbations could be successfully treated with prednisolone and/or calcimimetics, (f) the presence of CaSR autoantibodies that operated as biased allosteric modulators of CaSR, and (g) were likely to be conformational (i.e., recognizing and, thereby, stabilizing a unique active conformation of CaSR that activates Gq/11, activating phosphatidylinositol turnover, but not Gi/o). Our observations with these prominent commonalities may provide new insights into the phenotype and characteristics of AHH and the mechanisms by which the biased agonism of GPCRs operate.

Antenatal Glucocorticoid Administration Promotes Cardiac Structure and Energy Metabolism Maturation in Preterm Fetuses.

Although the rate of preterm birth has increased in recent decades, a number of preterm infants have escaped death due to improvements in perinatal and neonatal care. Antenatal glucocorticoid (GC) therapy has significantly contributed to progression in lung maturation; however, its potential effects on other organs remain controversial. Furthermore, the effects of antenatal GC therapy on the fetal heart show both pros and cons. Translational research in animal models indicates that constant fetal exposure to antenatal GC administration is sufficient for lung maturation. We have established a premature fetal rat model to investigate immature cardiopulmonary functions in the lungs and heart, including the effects of antenatal GC administration. In this review, we explain the mechanisms of antenatal GC actions on the heart in the fetus compared to those in the neonate. Antenatal GCs may contribute to premature heart maturation by accelerating cardiomyocyte proliferation, angiogenesis, energy production, and sarcoplasmic reticulum function. Additionally, this review specifically focuses on fetal heart growth with antenatal GC administration in experimental animal models. Moreover, knowledge regarding antenatal GC administration in experimental animal models can be coupled with that from developmental biology, with the potential for the generation of functional cells and tissues that could be used for regenerative medical purposes in the future.

Aldosterone-Producing Adenomas of Increased Size Are Associated With Higher Steroidogenic Activity.

CONTEXT: There are inconsistent results and insufficient evidence as to whether an association exists between the size and aldosterone-producing ability of aldosterone-producing adenomas. OBJECTIVE: We further investigated this possible association retrospectively. METHODS: A total of 142 cases of primary aldosteronism diagnosed as unilateral by adrenal venous sampling at 2 referral centers between 2009 and 2019 were included. We classified these individuals into small and large tumor groups using a diameter of 14 mm as a cutoff. This size was the median diameter of the tumor on the affected side of the adrenal gland. We compared plasma aldosterone concentration (PAC), plasma renin activity (PRA), PAC to PRA ratio, PAC from a saline infusion test (SIT), urinary aldosterone secretion (uAld), and serum potassium as indices of aldosterone-producing ability between the 2 groups. In some cases, we conducted histopathological evaluations and detection of the KCNJ5 mutation. RESULTS: PAC, PAC to PRA ratio, PAC from SIT, and uAld were higher and serum potassium was lower in the large tumor group. PAC, PAC from SIT, uAld, and serum potassium significantly correlated with tumor diameter. PRA was not associated with tumor diameter. Clear cell-dominant cases were more common in the large tumor group, while cases showing a strong expression of CYP11B2 were not significantly different between the groups. KCNJ5 mutations tended to be more common in the large tumor group. CONCLUSION: The higher aldosterone-producing ability in larger adenomas can be used to infer the responsible lesion and disease type.

Prenatal glucocorticoid administration enhances bilirubin metabolic capacity and increases Ugt1a and Abcc2 gene expression via glucocorticoid receptor and PXR in rat fetal liver.

AIM: Jaundice is especially common in premature infant born before 35 weeks. Because the premature infant liver is not fully developed at birth it may be incomplete the bilirubin metabolism. The purpose was to evaluate the metabolism and the excretion of bilirubin in the premature infant rat liver following prenatal glucocorticoid (GC) administration. METHODS: Dexamethasone (DEX) was administered subcutaneously to pregnant Wistar rats for two consecutive days on gestational days 17 and 19. The fetus were delivered by cesarean section in gestational days 19 and 21. The mRNA levels and protein levels of bilirubin-metabolic enzymes and transporters in the fetal liver tissues were analyzed using RT-PCR immunohistochemistry staining and ELISA, respectively. We evaluated that the effect of bilirubin-metabolic enzymes in the primary fetal rat hepatocytes treated with DEX after pretreated with glucocorticoid receptor (GR, Nr3c1) and Pxr (Nr1i2) siRNA. RESULTS: Ugt1a1 and Bsep (Abcb11) mRNA levels were significantly increased in the fetuses by prenatal GC administration. The mRNA levels of nuclear transcription factors Nr1i2, Car (Nr1i3), and Rxrα (Nr2b1) were also significantly increased in the fetuses by prenatal GC administration. In addition, DEX increased Nr1i2, Ugt1a1, and Abcc2 (Mrp2) mRNA levels in the primary fetal hepatocytes. The Nr3c1 or Nr1i2 siRNA-mediated knockdown suppressed the increases of Ugt1a1, and Abcc2 mRNA levels induced by DEX, indicating that DEX are mediated by GC receptor and PXR in primary fetal hepatocytes. CONCLUSIONS: These results suggest that prenatal GC administration increases bilirubin-metabolic ability, in the premature liver, which may prevent jaundice in neonates.

Prenatal glucocorticoid administration accelerates the maturation of fetal rat hepatocytes.

BACKGROUND: Prenatal glucocorticoid (GC) is clinically administered to pregnant women who are at risk of preterm birth for the maturation of cardiopulmonary function. Preterm and low-birth-weight infants often experience liver dysfunction after birth because their livers are immature. However, the effects of prenatal GC administration on the liver remain unclear. We aimed to investigate the effects of prenatal GC administration on the maturation of liver hepatocytes in preterm rats. METHODS AND RESULTS: Dexamethasone (DEX) was administered to pregnant Wistar rats on gestational days 17 and 19 before cesarean section. Real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed to determine the mRNA levels of albumin, hepatocyte nuclear factor-4 alpha (HNF4α), hepatocyte growth factor (HGF), thymus cell antigen 1 (Thy-1), cyclin B, and Cyclin-dependent kinase 1 (CDK1) in the liver samples. Immunohistochemical staining and enzyme-linked immunosorbent assay were performed to examine protein production. The hepatocytes enlarged because of growth and prenatal DEX administration. Albumin, HNF4α, and HGF levels increased secondary to growth and prenatal DEX administration. The levels of the cell cycle markers cyclin B and CDK1 gradually decreased during growth and with DEX administration. CONCLUSIONS: The results suggest that prenatal GC administration leads to hepatocyte maturation via expression of HNF4α and HGF in preterm fetuses.

Identification of interleukin-16 production on tumor aggravation in hepatocellular carcinoma by a proteomics approach.

BACKGROUND: Cytokines play an important role in the immune response, angiogenesis, cell growth, and differentiation in hepatocellular carcinoma (HCC). OBJECTIVE: We performed a comprehensive study to identify tumor-related cytokines and pathways involved in HCC pathogenesis. METHODS: Cytokine production was evaluated in human HCC tissues and adjacent non-tumor tissues using an antibody-based protein array technique. We compared cytokine expression in HCC tissues with that of hepatic hemangioma (HH), liver metastasis of colorectal cancer, and noncancerous liver tissues from transplantation donors. The protein levels and localization of the candidate cytokines were analyzed by western blotting and immunohistochemistry. RESULTS: Increased expression of interleukin (IL)-1 receptor antagonist, macrophage migration inhibitory factor, and IL-16 was observed in HCC and paired adjacent non-tumor tissues compared with noncancerous livers. In addition, there were increased IL-16 levels in HCC tissues compared with HH. IL-16 treatment significantly increased cell proliferation in vitro. The expression of extracellular signal-regulated kinase (ERK)1/2 and cyclin D1 was markedly increased in cells from two HCC cell lines, Huh7 and HepG2, in a dose- and time-dependent manner. Phosphorylated to total ERK1/2 ratio was increased in Huh7 cells following IL-16 50 ng/ml, but not HepG2 cells. ERK phosphorylation have occurred earlier than protein accumulation at 48 h. Pretreatment with the ERK inhibitor, FR18024, or an anti-IL-16 antibody reduced the increase in IL-16 production in HCC cells. CONCLUSIONS: These results suggest that cell proliferation induced by IL-16 is mediated through the ERK pathway, thus, we identified a new factor associated with HCC tumor growth.

Immune checkpoint inhibitor combination therapies very frequently induce secondary adrenal insufficiency.

Immune checkpoint inhibitors (ICIs) are potent therapeutic options for many types of advanced cancer. The expansion of ICIs use however has led to an increase in immune-related adverse events (irAEs). Secondary adrenal insufficiency (AI) can be life-threatening especially in patients with delayed diagnosis. We retrospectively investigated secondary AI in ICI-treated patients. A total of 373 cancer patients treated with ICIs were included and evaluated. An adrenocorticotropic hormone (ACTH) deficiency was described in 13 patients. Among 24 patients with a combination of nivolumab and ipilimumab therapy, 7 patients (29%) developed secondary AI in a median time of 8 weeks during the combination therapy and 2 of 15 patients (13%) developed isolated ACTH deficiency during maintenance nivolumab monotherapy following the combination therapy. More than half of the patients (4/7) with a combination therapy-induced multiple anterior hormone deficiencies was diagnosed as secondary AI based on regular ACTH and cortisol tests with slight subjective symptoms. Secondary AI can arise frequently and rapidly in cancer patients receiving a combination ICI therapy, and thus we speculate active surveillance of AI using regular ACTH and cortisol tests during the combination therapy might be useful for avoiding life-threatening conditions due to secondary AI.

V2 vasopressin receptor mutations.

V2 vasopressin receptor (V2R) is a member of the G protein-coupled receptor (GPCR) family in which many disease-causing mutations have been identified and thus generated much interest. Loss-of-function V2R mutations cause nephrogenic diabetes insipidus (NDI) whereas gain-of-function mutations cause nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The mechanisms underlying a V2R loss-of-function can be theoretically classified as either protein expression, localization (ER retention) or functional disorders. Functional analyses have revealed however that these mechanisms are likely to be complex. Strikingly, V2R mutations at the same site can result in opposite phenotypes, e.g., R137H and R137L/C cause NDI and NSIAD, respectively. These findings support the notion that the constitutive activation of GPCRs might be often associated with their instability and denaturation. Thus, functional analysis of disease-causing V2R mutations may not only reveal potential new treatment strategies using pharmacochaperones for NDI and inverse agonists for NSIAD, but also provide a greater understanding of the physiological functions of GPCRs and highlight the new paradigms, i.e., biased agonism and protean agonism.

Effectiveness and safety of cinacalcet for primary hyperparathyroidism: a single center experience.

Primary hyperparathyroidism (PHPT) is a common endocrine disease. Although surgical treatment is curative in most cases, there are few alternative therapies for the hypercalcemia caused by PHPT. Cinacalcet is a positive allosteric modulator of the calcium sensing receptor and was conditionally approved in Japan in 2014 to treat PHPT cases. However, there have been few reports on the outcomes. In our present study, we investigated the efficacy and safety of cinacalcet in 61 PHPT patients who were treated with this agent at our hospital between January 2014 and March 2017. The corrected serum Ca and intact PTH levels were significantly reduced by this treatment, whereas the serum phosphorus levels significantly increased. There were no significant differences in the eGFR or urinary Ca to urinary creatinine ratio between baseline and the maintenance phase. In terms of bone mineral density, there were significant increases observed in the 16 cases for whom a baseline value was available, 11 of whom had been treated for osteoporosis. The most common adverse events from cinacalcet treatment were gastrointestinal symptom, such as nausea and appetite loss. Other adverse events included severe dehydration due to hypercalcemia, myalgia, hypocalcemia, and increased urinary calcium excretion. Seven patients were switched to surgical treatment, and the drug was discontinued in 9 other patients, due to adverse effects. Our present study findings demonstrate that cinacalcet is an effective therapeutic option for PHPT from the perspective of hypercalcemia improvement but that adverse gastrointestinal effects of this drug occur at a frequency of about 10%.

Cinacalcet corrects biased allosteric modulation of CaSR by AHH autoantibody.

Biased agonism is a paradigm that may explain the selective activation of a signaling pathway via a GPCR that activates multiple signals. The autoantibody-induced inactivation of the calcium-sensing receptor (CaSR) causes acquired hypocalciuric hypercalcemia (AHH). Here, we describe an instructive case of AHH in which severe hypercalcemia was accompanied by an increased CaSR antibody titer. These autoantibodies operated as biased allosteric modulators of CaSR by targeting its Venus flytrap domain near the Ca2+-binding site. A positive allosteric modulator of CaSR, cinacalcet, which targets its transmembrane domain, overcame this autoantibody effect and successfully corrected the hypercalcemia in this patient. Hence, this is the first study to our knowledge that identifies the interaction site of a disease-causing GPCR autoantibody working as its biased allosteric modulator and demonstrates that cinacalcet can correct the AHH autoantibody effects both in vitro and in our AHH patient. Our observations provide potentially new insights into how biased agonism works and how to design a biased allosteric modulator of a GPCR. Our observations also indicate that the diagnosis of AHH is important because the severity of hypercalcemia may become fatal if the autoantibody titer increases. Calcimimetics may serve as good treatment options for some patients with severe AHH.

Molecular analysis and literature-based hypothesis of an immunonegative prostate small cell carcinoma causing ectopic ACTH syndrome.

Ectopic ACTH syndrome (EAS) due to a prostate small cell carcinoma (SCC) is very rare with only 26 cases reported to date and has a poor prognosis. We here describe another case of this disorder that was clinically typical based on prior reports as it showed hypercortisolemia and severe hypokalemia with multiple metastasis. However, our current case of prostate SCC causing EAS is the first to display negative immunostaining for ACTH despite detectable POMC mRNA expression in the primary lesion. ACTH immunonegativity is thought to be associated with a more aggressive disease course and a poorer prognosis although there are few studies of the underlying mechanisms. We explored two possibilities for this finding in our current patient: aberrant POMC processing prevented immunodetection with an anti-ACTH antibody; and the ACTH content per cell was below the threshold for immunodetection due to its rapid secretion or low synthesis. The aberrant processing theory was thought to be less likely because of immunonegative findings even using anti-POMC/ACTH antibodies. As the plasma ACTH levels in our patient were comparable with those reported for previous immunopositive prostate EAS cases, we speculated that the depletion of ACTH may be caused not only by rapid secretion but also by low production levels as a sign of de-differentiation. De-differentiation may therefore explain the mechanism underlying the negative correlation between immunoreactivity for ACTH in EAS and disease aggressiveness. We believe that our present findings will be of use in future prospective studies aimed at confirming the mechanism of immunonegativity.

Bexarotene-induced hypothyroidism: Characteristics and therapeutic strategies.

OBJECTIVE: Central hypothyroidism (CH) is a well-known adverse effect of bexarotene treatment for cutaneous T-cell lymphoma (CTCL). While concomitant levothyroxine therapy is recommended in these cases, associations between ethnic variation or susceptibility and bexarotene-induced CH have not yet been reported. This study aimed to characterize the kinetics and dose dependency of bexarotene-induced CH in Japanese patients. DESIGN AND PATIENTS: Sixty-six Japanese patients with CTCL were retrospectively investigated by evaluating thyroid function during the early phase of bexarotene therapy. RESULTS: At one week after bexarotene initiation, TSH and FT4 values significantly declined. However, this effect was not bexarotene dose-dependent at least at the dose of 96-320 mg/m2 . Approximately 1 month later, 61 patients exhibited hypothyroidism at a relatively low dose of bexarotene (average 251 mg/m2 /day). Forty-five study cases showed this effect at 1 week. Simple regression analyses indicated that higher pretreatment TSH values (at a cut-off value of 1.30:73% sensitivity, 57% specificity) or lower normal (within the lower half of the reference range) pretreatment FT4 values (84% sensitivity, 57% specificity) were predictive of hypothyroidism at 1 week. The remaining 21 cases showed euthyroidism at 1 week, at which TSH values may roughly predict their thyroid function at 1 month (at a cut-off value of 0.05:100% sensitivity, 80% specificity). CONCLUSIONS: Preventive treatment with levothyroxine is recommended for Japanese CTCL patients prior to bexarotene therapy. Minimally, it should be considered for patients with a pretreatment TSH above 1.30, a lower normal pretreatment FT4, or a TSH below 0.05 at 1 week.

Vasoactive intestinal peptide increases apoptosis of hepatocellular carcinoma by inhibiting the cAMP/Bcl-xL pathway.

Vasoactive intestinal peptide (VIP) is a modulator of inflammatory responses. VIP receptors are expressed in several tumor types, such as colorectal carcinoma. The study described herein was conducted to confirm the presence of VIP and its receptors (VPAC1 and VPAC2) in surgically resected hepatocellular carcinoma (HCC) tissues and in the HCC cell line Huh7. The mechanism responsible for apoptosis of HCC cells was then examined because VIP treatment (10-10  M) significantly suppressed proliferation of Huh7 cells. In examining apoptosis-related proteins, we found caspase-3 to be significantly increased and Bcl-xL and cyclic AMP (cAMP) response element-binding protein (CREB) to be significantly decreased in Huh7 cells cultured with VIP. Furthermore, the CREB level and phosphorylation were reduced. These effects were reversed by the addition of VIP receptor antagonist or cAMP antagonist Rp-cAMPS. Pretreatment with cAMP analogue blocked the increased apoptosis, suggesting that VIP induces apoptosis via a PKA-independent signaling mechanism. Our data indicate that VIP prevents the progression of HCC by apoptosis through the cAMP/Bcl-xL pathway.

Isolation of Adipose-Derived Stem/Stromal Cells from Cryopreserved Fat Tissue and Transplantation into Rats with Spinal Cord Injury.

Adipose tissue contains multipotent cells known as adipose-derived stem/stromal cells (ASCs), which have therapeutic potential for various diseases. Although the demand for adipose tissue for research use remains high, no adipose tissue bank exists. In this study, we attempted to isolate ASCs from cryopreserved adipose tissue with the aim of developing a banking system. ASCs were isolated from fresh and cryopreserved adipose tissue of rats and compared for proliferation (doubling time), differentiation capability (adipocytes), and cytokine (hepatocyte growth factor and vascular endothelial growth factor) secretion. Finally, ASCs (2.5 × 106) were intravenously infused into rats with spinal cord injury, after which hindlimb motor function was evaluated. Isolation and culture of ASCs from cryopreserved adipose tissue were possible, and their characteristics were not significantly different from those of fresh tissue. Transplantation of ASCs derived from cryopreserved tissue significantly promoted restoration of hindlimb movement function in injured model rats. These results indicate that cryopreservation of adipose tissue may be an option for clinical application.

Intravenous infusion of adipose-derived stem/stromal cells improves functional recovery of rats with spinal cord injury.

BACKGROUND AIMS: Adipose tissue has therapeutic potential for spinal cord injury (SCI) because it contains multipotent cells known as adipose-derived stem/stromal cells (ASCs). In this study, we attempted intravenous ASC transplantation in rats with SCI to examine the effect on functional recovery. METHODS: ASCs (2.5 × 106) were intravenously infused into SCI rats, after which hindlimb motor function was evaluated. Distribution of transplanted ASCs was investigated and growth factor/cytokine levels were determined. RESULTS: Intravenous transplantation of ASCs promoted the functional recovery in SCI rats and reduced the area of spinal cord cavitation. A distribution study revealed that ASCs gradually accumulated at the site of injury, but long-term survival of these cells was not achieved. Levels of growth factors increased only slightly in the spinal cord after ASC transplantation. Unexpectedly, cytokine-induced neutrophil chemoattractant (CINC)-1 showed a transient but substantial increase in the spinal cord tissue and blood of the ASC group. CINC-1 was secreted by ASCs in vitro, and the sponge implantation assay showed that CINC-1 and ASCs induced angiogenesis. CINC-1 promoted functional recovery in SCI rats, which was similar to the ASCs. Expression of glial cell line-derived neurotrophic factor was greater in the ASC group than in the CINC-1 group, although both promoted extracellular signal-regulated kinase (ERK)1/2 phosphorylation; Akt phosphorylation was enhanced in the spinal cord after ASC transplantation. CONCLUSIONS: Our findings indicated that intravenously transplanted ASCs gradually accumulated in the injured spinal cord, where cytokines such as CINC-1 activated ERK1/2 and Akt, leading to functional recovery.

Analysis of the V2 Vasopressin Receptor (V2R) Mutations Causing Partial Nephrogenic Diabetes Insipidus Highlights a Sustainable Signaling by a Non-peptide V2R Agonist.

Disease-causing mutations in G protein-coupled receptor (GPCR) genes, including the V2 vasopressin receptor (V2R) gene, often cause misfolded receptors, leading to a defect in plasma membrane trafficking. A novel V2R mutation, T273M, identified in a boy with partial nephrogenic diabetes insipidus (NDI), shows intracellular localization and partial defects similar to the two mutants we described previously (10). Although non-peptide V2R antagonists have been shown to rescue the membrane localization of V2R mutants, their level of functional rescue is weak. Interestingly, it has been reported that a non-peptide agonist, OPC51803, activates misfolded V2R mutants intracellularly without degradation, thus potentially serving as a therapeutic agent against NDI (14). In our current experiments, however, a peptide antagonist blocked arginine vasopressin (AVP)- or OPC51803-stimulated cAMP accumulation both in COS-7 and MDCK cells, suggesting that OPC51803 mainly stimulates cell surface V2R mutants. In addition, our analyses revealed that OPC51803 works not only as a non-peptide agonist that causes activation/ß-arrestin-dependent desensitization of V2R mutants expressed at the plasma membrane but also as a pharmacochaperone that promotes the endoplasmic reticulum-retained mutant maturation and trafficking to the plasma membrane. The ratio of the pharmacochaperone effect to the desensitization effect likely correlates negatively with the residual function of the tested mutants, suggesting that OPC5 has a more favorable effect on the V2R mutants with a less residual function. We speculated that the canceling of the desensitization effect of OPC51803 by the pharmacochaperone effect after long-term treatment may produce sustainable signaling, and thus pharmacochaperone agonists such as OPC51803 may serve as promising therapeutics for NDI caused by misfolded V2R mutants.

Inverse agonism: the classic concept of GPCRs revisited [Review].

In the classical two-state model, G protein-coupled receptors (GPCRs) are considered to exist in equilibrium between an active and an inactive conformation. Thus, even at the resting state, some subpopulation of GPCRs is in the active state, which underlies the basal activity of the GPCRs. In this review, we discuss inverse agonists, which are defined as GPCR ligands that shift the equilibrium toward the inactive state and thereby suppress the basal activity. Theoretically, if constitutive activation plays an essential role in the pathogenesis of a disease, only inverse agonists, and not neutral antagonists, can reverse this pathophysiological activation. Although many pharmacological examples of inverse agonism have been identified, its clinical importance is still unclear and debated. Thus, even though inverse agonism of angiotensin receptor blockers (ARBs) has been discussed for more than 10 years, its clinical relevance remains to be completely clarified.