A functional precision medicine 3D microtumor platform to identify and personalize novel indications

e15588

Background: We have previously validated our 3D microtumor platform that captures the tumor heterogeneity of a patient's primary tumor biopsy using a patient-derived xenograft model (Nikolov et al. ASCO 2019, e17076). Here we describe its utility to identify FDA approved therapies that may be effective for tumors previously not considered. Methods: A fresh tumor sample of a liver metastasis of a 45-year-old colorectal cancer patient was shipped overnight and processed to create hundreds of live 3D microtumors. These microtumors were treated with a panel of 12 commonly used drugs including chemotherapies and targeted therapies. Treatment effects were quantified and validated on fresh and cryopreserved/thawed samples using our metabolic and proprietary multiplexed fluorescent staining technologies to quantify the ratio of live and dead cells in those microtumors. Results: None of the conventional treatments in the 12-panel drug test suggested any efficacy as determined by the log of efficacy concentrations (in µmol/l): Oxaliplatin 2.59, 5-FU 3.00 (upper cut-off value), paclitaxel 1.72, topotecan 3.00, irinotecan (SN-38) 2.33, gemcitabine 2.44, and bevacizumab 3.00. The microtumors also appeared resistant to several targeted therapies that are not commonly given, ranging from 1.85 to 2.44. However, abemaciclib had an efficacy of 0.49, which was confirmed on thawed samples with all drug efficacy concentrations again suggesting resistance, except for abemaciclib at 0.67. Conclusions: Our 3D microtumor platform may be a useful tool a) to identify rescue treatment options for metastatic patients that have multi-resistant tumors and b) to identify novel indications to personalize FDA-approved cytotoxic or targeted therapies as a companion diagnostic.

PDX validation of a 3D microtumor platform

3029

Title: Patient-derived xenograft validation of a 3D microtumor platform Background: Patient-derived xenograft (PDX) mouse models are thought to most closely reflect the biology of a patient’s cancer. Unfortunately, growing sufficient tumor in a PDX model takes several months and more often than not, the tumor fails to grows at all. The SAGE Direct Platform, an in-vitro model, can create hundreds of live microtumors from virtually every patient’s viable biopsy and test a panel of clinically relevant drugs within no more than 1 week. Thus, concordance of results from a PDX model with results of the SAGE Direct Platform would support a rational for the platform to be potentially useful to predict tumor response in cancer patients. Methods: A bladder cancer from a 77 year old female was used to establish a PDX model. Mice were divided into three groups receiving either saline (control), cisplatin, or gemcitabine intraperitoneal on the days 1, 8, and 13, and tumor growth was observed. One tumor sample was used to create 3D microtumors and those were tested using the same drugs. Results: Tumor growth (exceeding 1,000 mm3) was similar after cisplatin compared to control (4.8 vs. 3.7 weeks). After gemcitabine tumors initially shrank and only started growing a couple of weeks after the end of treatment so that 1,000 mm3 was only reached after 10.2 weeks (p<0.001 compared to cisplatin and control). In the SAGE Direct Platform the EC50 of cisplatin was 97.3 µM and thus two orders of magnitudes higher than the EC50 of gemcitabine, which was 0.7 µM. Conclusions: Both the PDX model and the SAGE Direct Platform have shown this bladder cancer to be virtually resistant to cisplatin while very sensitive to gemcitabine. The next steps of these preliminary data could be to repeat this experimental design with other tumors and/or to start an observational cohort study in patients correlating the SAGE Direct Platform results to patient outcomes.

A 3D micro-tumor model capturing tumor heterogeneity of a patient's primary tumor biopsy

e17076

Background: Cancer is a genetic disease. However, because of the randomness of mutations, less than 20% of patients have known driver mutations and it is estimated that in the United States, less than 5% of patients benefit from genome-driven targeted therapy. [1] Thus, treatment for millions of cancer patients is still today a hit or miss experiment. Therefore, we have developed a developed a genome-free and mechanism agnostic 3D micro-tumor platform (SAGE Direct Test™) to help oncologists and patients alike to directly identify the treatment that is most effective for an individual patient. Methods: The SAGE Direct Test™ works as follows with actionable results within 1 week: Disaggregation of the patient’s tumor tissue and recreation of 3D microtumors that resemble the tumor biology and heterogeneity of the patient’s primary tumor biopsy. Exposure of those 3D microtumors to a panel of potential treatment options. Measuring the metabolic or image-based response of such 3D microtumors, for example as EC50. Results: We created hundreds of 3D micro-tumors from biopsies of 7 ovarian cancer patients. Not surprisingly, when these microtumors were exposed to various cytotoxic chemotherapies such as cisplatin, paclitaxel or gemcitabine or targeted therapies such as erlotinib or crizotinib, the median EC50 varied significantly from drug to drug. However, for any given drug, the EC50 varied significantly from patient to patient, with an interquartile range spreading often by more than one order of magnitude. Values in the table are standardized means with their standard deviations. Conclusions: We have developed a 3D micro-tumor model that can measure the sensitivity of a patient’s cancer biopsy tissue within one week. Furthermore, we found that the sensitivities of those 3D microtumors differed significantly from patient to patient. These results have not yet established a correlation between this 3D micro-tumor model and patients’ tumor responses, which is one of the next steps towards establishing clinical utility for the millions of patients where genomic testing remains non-informative. [Table: see text]

Evaluation of a High-Throughput Fluorescence Assay Method for hERG Potassium Channel Inhibition

The number of projects in drug development that fail in late phases because of cardiac side effects such as QT prolongation can impede drug discovery and development of projects. The molecular target responsible for QT prolongation by a wide range of pharmaceutical agents is the myocardial hERG potassium channel. It is therefore desirable to screen for compound interactions with the hERG channel at an early stage of drug development. Here, the authors report a cell-based fluorescence assay using membrane potential-sensitive fluorescent dyes and stably transfected hERG channels from CHO cells. The assay allows semiautomated screening of compounds for hERG activity on 384-well plates and is sufficiently rapid for testing a large number of compounds. The assay is robust as indicated by a Z′ factor larger than 0.6. The throughput is in the range of 10,000 data points per day, which is significantly higher than any other method presently available for hERG. The data obtained with the fluorescence assay were in qualitative agreement with those from patch-clamp electrophysiological analysis. There were no false-positive hits, and the rate of false-negative compounds is currently 12% but might be further reduced by testing compounds at higher concentration. Quantitative differences between fluorescence and electrophysiological methods may be due to the use- or voltage-dependentactivity of the antagonists.

Anesthesia management of patients undergoing hyperthermic isolated limb perfusion with melphalan for melanoma treatment: an analysis of 17 cases

Background

Hyperthermic isolated limb perfusion (HILP) is used for patients with intractable or extensive in-transit metastatic melanoma of the limb to deliver high concentrations of cytotoxic agents to the affected limb and offers a treatment option in a disease stage with a poor prognosis when no treatment is given.

Methods

In a retrospective chart review of 17 cases, we studied the anesthetic and hemodynamic changes during HILP and its management.

Results

HILP was well tolerated except in one case that is described herein. We present summary data of all cases undergoing upper and lower limb perfusion, discuss our current clinical practice of preoperative, perioperative and intraoperative patient care including the management of HILP circuit.

Conclusion

HILP is a challenging procedure, and requires a team effort including the surgical team, anesthesia care providers, perfusionists and nurses. Intraoperatively, invasive hemodynamic and metabolic monitoring is indispensable to manage significant hemodynamic and metabolic changes due to fluid shifts and release of cytokines.

Accuracy and clinical utility of in vitro cytometric profiling to personalize chemotherapy: Preliminary findings of a systematic review and meta-analysis

e22188

Background: Cytometric analysis, or in-vitro functional profiling, has been developed as a method to predict tumor response to different drugs with the premise to personalize chemotherapy and improve patient outcomes. Methods: We performed a systematic review and a meta-analysis a) of correlative studies using cytometric profiling that reported diagnostic accuracy (sensitivity and specificity) and b) of effectiveness studies comparing patient outcomes when allocated to treatment guided by a cytometric assay versus population-based standard of care. We used Meta-DiSc software to find pooled sensitivity and specificity and analyze the summary receiver operating characteristic (sROC) curve and used Review Manager 5.1 to generate forest plots on overall tumor response (50% or greater decrease in tumor diameter) and on 1-year overall survival. Results: We included 28 mostly retrospective trials (n=664) reporting accuracy data and 15 prospective trials (n=1917) reporting therapeutic efficacy data. The accuracy of correlative study revealed an overall sensitivity of 0.922 (95% confidence interval 0.888 to 0.948), specificity of 0.724 (95% CI 0.669 to 0.774) and an area under the sROC curve of 0.893 (SE=0.023, p<0.001). Studies comparing the clinical utility revealed a two-fold overall tumor response for an assay-guided therapy versus standard of care therapy (odds ratio 2.04, 95% CI 1.62 to 2.57, p<0.001). Similarly, patients who received assay-guided therapy compared to those who received standard of care or physician’s choice had a significantly higher 1-year survival rate (OR 1.44, 95% CI 1.06 to 1.95, p=0.02). Conclusions: Despite various limitations of individual studies, the aggregate and fairly consistent evidence of these data suggests cytometric profiling to be accurate, to improve overall tumor response, and to increase 1-year patient survival. Given the enormous potential for our society, a well-designed and sufficiently-powered randomized controlled trial is urgently needed to validate these results.

A randomized, double-blind study to evaluate the efficacy and safety of three different doses of palonosetron versus placebo in preventing postoperative nausea and vomiting over a 72-hour period

BACKGROUND

We designed this multicenter, randomized, double-blind study to assess the efficacy and safety of three doses of palonosetron, compared with placebo, on the incidence and severity of postoperative nausea and vomiting (PONV) in inpatients for 72 h after surgery.

METHODS

Female patients undergoing either elective gynecological or breast surgery were stratified according to two additional PONV risk factors: nonsmoking status and history of PONV and/or motion sickness. Five hundred forty-four patients with one or both of these risk factors were randomized to receive one of the three doses of IV palonosetron (0.025 mg, 0.050 mg, 0.075 mg) or placebo immediately before induction of anesthesia. The primary efficacy end-point was complete response (CR: no emesis and no use of rescue medications) evaluated at the 0-24 and 24-72 h time intervals after surgery.

RESULTS

CR rates for placebo and palonosetron 0.075 mg were 36% and 56% for 0-24 h (P = 0.001), 52% and 70% for 24-72 h (P = 0.002) and 36% and 52% (P = 0.010) for the 0-72 h postoperative interval. Palonosetron 0.075 mg was associated with less intense nausea (e.g., toward "mild" or "none") versus placebo during the 0-24 h (P < 0.001) time interval and significantly delayed median time to emesis (P = 0.002) and treatment failure (P = 0.004). Although CR rates for both the 0.025 mg and 0.050 mg palonosetron doses were not statistically superior to placebo for the 0-24 h or 24-72 h periods, both lower doses reduced nausea severity during the 0-24 h period (P = 0.040 and P = 0.004).

CONCLUSION

A single 0.075-mg IV dose of palonosetron effectively reduced the severity of nausea and delayed the time to emesis and treatment failure in the inpatient surgical setting; lower doses were not as effective.

Preventing postoperative nausea and vomiting: post hoc analysis of pooled data from two randomized active-controlled trials of aprepitant

OBJECTIVE

Compared with the 5HT(3) antagonist ondansetron, the NK(1) antagonist aprepitant has been shown in two double-blind trials to provide greater protection against postoperative vomiting and comparable or greater control of nausea. Post hoc analyses of pooled data from these trials were performed to more fully characterize the efficacy profile of aprepitant in terms of nausea and use of rescue therapy.

RESEARCH DESIGN AND METHODS

Patients (n = 1599) scheduled for major surgery under general anesthesia (primarily gynecological surgery) were assigned to receive a preoperative dose of aprepitant 40 mg PO, 125 mg PO, or ondansetron 4 mg IV. in two randomized, double-blind, clinical trials.

MAIN OUTCOME MEASURES

Post-surgery vomiting episodes, use of rescue therapy, and nausea severity (verbal rating scale).

RESULTS

In the 24 hours after surgery, aprepitant 40 mg was more effective than ondansetron for all five endpoints evaluated: (1) no significant nausea (56.4% vs. 48.1%); (2) no nausea (39.6% vs. 33.1%); (3) no vomiting (86.7% vs. 72.4%); (4) no nausea and no vomiting (38.3% vs. 31.4%); and (5) no nausea, no vomiting, and no use of rescue (37.9% vs. 31.2%) (p < 0.035 for the odds ratio for each comparison). Numerically more patients receiving aprepitant 125 mg also achieved these endpoints compared with ondansetron.

CONCLUSIONS

These post hoc analyses confirm the favorable efficacy profile of aprepitant for the prevention of post operative nausea and vomiting.

Effects of nitrous oxide on intraoperative bowel distension

PURPOSE OF REVIEW

To evaluate evidence and present an objective view on the effect of nitrous oxide on bowel function.

RECENT FINDINGS

We determined in a metaanalysis that the chance of having intraoperative bowel distension was increased about two fold in patients receiving nitrous oxide as the anaesthetic carrier gas compared with those receiving nitrogen or oxygen. In a separate logistic regression analysis, we found that the duration of nitrous oxide exposure was a key factor in explaining this difference, as well as the variability of previously published data. In contrast to bowel distension, surgical operating conditions were not hindered by the use of nitrous oxide; however, the number of patients analysed for operating conditions was much less than that for bowel distension data.

SUMMARY

Nitrous oxide causes clinically and statistically recognizable bowel distension. This distension, however, does not always exacerbate the surgical conditions, nor does it appear to delay bowel movement or hospital discharge. Although no major problems were noted in bowel functions other than distension, we recommend avoiding nitrous oxide administration during prolonged bowel operations.

2-(2-Oxo-1,4-dihydro-2H-quinazolin-3-yl)- and 2-(2,2-dioxo-1,4-dihydro-2H-2lambda6-benzo[1,2,6]thiadiazin-3-yl)-N-hydroxy-acetamides as potent and selective peptide deformylase inhibitors

Potent, selective, and structurally new inhibitors of the Fe(II) enzyme Escherichia coli peptide deformylase (PDF) were obtained by rational optimization of the weakly binding screening hit (5-chloro-2-oxo-1,4-dihydro-2H-quinazolin-3-yl)-acetic acid hydrazide (1). Three-dimensional structural information, gathered from Ni-PDF complexed with 1, suggested the preparation of two series of related hydroxamic acid analogues, 2-(2-oxo-1,4-dihydro-2H-quinazolin-3-yl)-N-hydroxy-acetamides (A) and 2-(2,2-dioxo-1,4-dihydro-2H-2lambda(6)-benzo[1,2,6]thiadiazin-3-yl)-N-hydroxy-acetamides (B), among which potent PDF inhibitors (37, 42, and 48) were identified. Moreover, two selected compounds, one from each series, 36 and 41, showed good selectivity for PDF over several endoproteases including matrix metalloproteases. However, these compounds showed only weak antibacterial activity.

Hydroxamic acid derivatives as potent peptide deformylase inhibitors and antibacterial agents

Low-molecular-weight beta-sulfonyl- and beta-sulfinylhydroxamic acid derivatives have been synthesized and found to be potent inhibitors of Escherichia coli peptide deformylase (PDF). Most of the compounds synthesized and tested displayed antibacterial activities that cover several pathogens found in respiratory tract infections, including Chlamydia pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. The potential of these compounds as antibacterial agents is discussed with respect to selectivity, intracellular concentrations in bacteria, and potential for resistance development.

Transnasal transesophageal echocardiography: a modified application mode for cardiac examination in ventilated patients

In 42 endotracheally intubated patients, we examined the utility of a miniaturized monoplane probe for transnasal transesophageal echocardiography (TEE). Transnasal TEE was prospectively evaluated in 26 deeply and 16 mildly sedated patients receiving topical anesthesia with lidocaine jelly 2%. The patients with deep sedation were additionally examined with transoral monoplane and multiplane TEE. Transnasal esophageal insertion of the TEE probe was successfully performed in 90% of patients. Endotracheal malpositioning was corrected in two patients. Nasal bleeding required treatment in another patient. Topical anesthesia was adequate in 82% of mildly sedated patients. Left ventricular short- and four-chamber long-axis views of good quality were obtained with transnasal (transoral) monoplane TEE in 76% (81%) and 92% (96%) of patients (differences not significant). Compared with conventional multiplane TEE, transnasal monoplane TEE missed diagnoses in 19% of patients. The relative error (mean +/- SEM) of quantification with transnasal TEE was <9% +/- 2% for ventricular diameters and <7% +/- 2% for cross-sectional area measurements, with a bias of 0.5 +/- 3.8 cm2 and 0.1 +/- 2.4 cm2 (mean +/- 2 SD) for left ventricular end-diastolic and end-systolic short-axis areas. The relative error in measuring intracardiac flow velocities was >40%, but systolic to diastolic peak velocity ratios at the valvular site were determined with an error <4% +/- 3%. Transnasal monoplane TEE can be performed even in mildly sedated patients with an endotracheal tube without further need for analgesia or sedation. The technique is as useful as conventional transoral TEE to image standard tomographic planes for quantification, but it is less suited for comprehensive echocardiographic diagnosing.

IMPLICATIONS

Transnasal insertion of a miniaturized monoplane transesophageal echocardiography (TEE) probe was studied in endotracheally intubated patients. Nasal passage was well tolerated even by patients with only mild sedation. Imaging quality was similar to conventional transoral monoplane TEE with larger transducers, but technical restraints cause a deficit in complete cardiac diagnosing obtained with multiplane TEE.

RARalpha antagonist Ro 41-5253 inhibits proliferation and induces apoptosis in breast-cancer cell lines

Ro 41-5253 is a RARalpha-selective antagonist that binds RARalpha but does not induce transcriptional activation and does not influence RAR/RXR heterodimerization and DNA binding. This retinoid inhibits proliferation and induces apoptosis in MCF-7 and ZR-75.1 estrogen-receptor-positive breast-carcinoma cells in a dose-dependent way. The anti-proliferative effect is more evident in ZR-75.1 cells than in MCF-7 cells and is probably mediated by anti-AP1 activity, a mechanism known to be implied in the action of several retinoids. In the induction of apoptosis also ZR-75.1 cells are more sensitive to treatment with Ro 41-5253 than MCF-7 cells. In ZR-75.1 cells an apoptotic/hypodiploid DNA peak is already evident after 2 days of incubation, whereas in MCF-7 cells it appears only after 4 days. The highest percentage of apoptotic cells, for both cell lines, is reached after 6 days of treatment. The apoptosis pathway is p53-independent and bcl-2 downregulation seems to be correlated with an increase in TGF-beta1 protein. The MDA-MB-231 estrogen-receptor-negative cell line is poorly responsive to Ro 41-5253 treatment, both in terms of proliferation inhibition and apoptosis induction. Ro 41-5253 has proliferation-inhibiting and apoptosis-inducing properties that are not mediated by transcriptional activation from retinoic-acid response elements. This retinoid antagonist seems to be a compound that exerts an anti-tumor activity but does not induce the toxic side effects of retinoids and might, therefore, be considered as a candidate for cancer therapy.

The relation between left ventricular wall stress shortening and preload changes in ventilated patients

The relation between left ventricular end-systolic wall stress (ESWS) and the velocity of circumferential fibre shortening (Vcfs) was examined in four non-random groups with 12 patients each. In group A, preload was increased by the administration of hydroxyethylstarch 30 mL min-1. In group B, preload was reduced by administering nitroglycerine 4.0 +/- 0.8 mg h-1. A change in pulmonary capillary wedge pressure by 3 mmHg was taken to be the end point for preload manipulation. To assess a change in the relation between ESWS and Vcfs, patients in group C received adrenaline at an infusion rate of 4 g min-1. Patients in group D served as the controls. Geometric variables of ESWS and Vcfs were determined by transoesophageal echocardiography. A linear model was used to assess the relation between ESWS and Vcfs within each group by regression analysis, and analysis of covariance performed to detect significance of intragroup and intergroup differences (P < 0.005). No significant changes were found during preload intervention. With adrenaline, Vcfs increased significantly for a given ESWS. It is concluded that the relation between ESWS and Vcfs, in a multiple patient setting, is independent of modest preload changes and may have the potential to indicate inotropic effects.

Relation of echocardiographic preload indices to stroke volume in critically ill patients with normal and low cardiac index

OBJECTIVE

To examine the usefulness of preload indices obtained by transoesophageal echocardiography (TOE) for estimating stroke volume at various levels of cardiac index.

DESIGN

Prospective clinical study.

SETTING

Intensive care unit with surgical patients.

PATIENTS

16 ventilated patients monitored via Swan-Ganz catheterization and TOE.

INTERVENTIONS

Echocardiographic images of left ventricular cross-sectional short-axis areas were analysed for the preload indices end-diastolic area (EDA), stroke area and end-diastolic wall stress. The relation between these indices and stroke volume, calculated from thermodilution cardiac output, was analysed in all patients and in nine patient groups discriminated by various ranges in heart rate (< or = 70 to > 110 beats/min), pulmonary artery occlusion pressure (< or = 8 to > 12 mmHg) and cardiac index (< or = 3.0 to > 4.2 l/min per m2).

MEASUREMENTS AND RESULTS

Overall stroke volume (n = 155) correlated significantly (p < 0.0001) with EDA (r = 0.89) and stroke area (r = 0.80). The correlation with end-diastolic wall stress was non-significant (r = 0.51). Linearity in the relation between stroke volume and EDA or stroke area was independent of variations in heart rate and pulmonary artery occlusion pressure. Stroke volume correlated well with EDA and stroke area, when cardiac index was normal or high, but the relation slightly deteriorated (r = 0.63 to < or = 0.72) when the cardiac index was low. Changes in EDA and stroke area by more than 1, 2 or 3 cm2 were weak predictors for changes in stroke volume greater than 20%.

CONCLUSIONS

Stability of the relation between echocardiographic preload indices and stroke volume emphasize the potential of TOE for continuous preload monitoring in the critically ill.

Comparative disposition, receptor affinity, and teratogenic activity of sulfon arotinoids

To investigate the relationship between sulfon arotinoid biotransformation and teratogenic activity, the potency of the ethyl (Ro 15-1570) and methyl (Ro 14-9706) arotinoid sulfones and their in vivo disposition in pregnant hamsters were studied. Administration of Ro 15-1570 was teratogenic, but Ro 14-9706 showed no such activity. Total absorbed doses of the ethyl and methyl sulfones (measured as maternal plasma AUC) were very similar. Total delivered dose of Ro 14-9706 to liver and lung was 120-160% that of Ro 15-157, and Ro 14-9706 was transferred in greater amounts to the embryo as well. Placenta AUC for parent sulfon arotinoids was 160-250% that in the embryo. Plasma analyses by HPLC suggested that the ethyl sulfone was oxidized and appeared in maternal plasma as the corresponding sulfinic (Ro 14-9572) and sulfonic (Ro 14-3899) acids, amounting to 10% and 16%, respectively, of the mean maternal ethyl sulfone Cmax value. The concentrations of sulfinic and sulfonic metabolites were always less than the analytical limit of detection in placenta and embryo after maternal ethyl sulfone intubation. Neither the sulfinic nor the sulfonic acid were ever detected in maternal circulation, placenta, or embryo after methyl sulfone intubation. Comparisons of their binding affinities found that neither the ethyl nor the methyl arotinoid sulfone could act as a ligand for cellular retinoic acid-binding protein (CRABP), nor could these compounds bind retinoid nuclear receptors (RAR). Transcriptional activation of RARs was weak and similar for both compounds. The sulfinic and sulfonic acid arotinoids bind and transactivate RARs, and bind CRABP with efficiencies similar to all-trans-retinoic acid. Furthermore, they are active in cultured limb bud chondrocytes. The results suggest that the methyl sulfone (in accord with its lack of activity in cultured limb bud chondrocytes) is of no toxicologic significance in hamster embryo--even after relatively high delivered dose. Teratogenicity of the ethyl sulfone (which shows marked inhibition of chondrogenesis in cultured limb bud) does not appear to depend on measurable concentrations of these sulfinic/sulfonic acid metabolites in the hamster embryo.

Different agonist- and antagonist-induced conformational changes in retinoic acid receptors analyzed by protease mapping

The pleiotropic effects of retinoic acid on cell differentiation and proliferation are mediated by two subfamilies of nuclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). Recently the synthetic retinoid Ro 41-5253 was identified as a selective RAR alpha antagonist. As demonstrated by gel retardation assays, Ro 41-5253 and two related new RAR alpha antagonists do not influence RAR alpha/RXR alpha heterodimerization and DNA binding. In a limited trypsin digestion assay, complexation of RAR alpha with retinoic acid or several other agonistic retinoids altered the degradation of the receptor such that a 30-kDa proteolytic fragment became resistant to proteolysis. This suggests a ligand-induced conformational change, which may be necessary for the interaction of the DNA-bound RAR alpha/RXR alpha heterodimer with other transcription factors. Our results demonstrate that antagonists compete with agonists for binding to RAR alpha and may induce a different structural alteration, suggested by the tryptic resistance of a shorter 25-kDa protein fragment in the digestion assay. This RAR alpha conformation seems to allow RAR alpha/RXR alpha binding to DNA but not the subsequent transactivation of target genes. Protease mapping with C-terminally truncated receptors revealed that the proposed conformational changes mainly occur in the DE regions of RAR alpha. Complexation of RAR beta, RAR gamma, and RXR alpha, as well as the vitamin D3 receptor, with their natural ligands resulted in a similar resistance of fragments to proteolytic digestion. This could mean that ligand-induced conformational changes are a general feature in the hormonal activation of vitamin D3 and retinoid receptors.

A retinoic acid receptor alpha antagonist selectively counteracts retinoic acid effects

Retinoic acid (RA) exerts its pleiotropic effects on cell growth and differentiation through the activation of a family of transcription factors-the RA receptors (RARs). Three subtypes of these receptors exist, RAR alpha, RAR beta, and RAR gamma. The receptors are differentially expressed in different cell types and stages of development, suggesting that they may regulate different sets of genes. We have identified a synthetic retinoid with the characteristics of a selective RAR alpha antagonist. This antagonist counteracts RA effects on HL-60 cell differentiation and on B-lymphocyte polyclonal activation. Beyond its potential practical relevance, this and other specific antagonists will be useful to dissect the RAR system and to assign to one given receptor each of the many RA-regulated functions.