A selective and orally bioavailable VHL-recruiting PROTAC achieves SMARCA2 degradation in vivo

Targeted protein degradation offers an alternative modality to classical inhibition and holds the promise of addressing previously undruggable targets to provide novel therapeutic options for patients. Heterobifunctional molecules co-recruit a target protein and an E3 ligase, resulting in ubiquitylation and proteosome-dependent degradation of the target. In the clinic, the oral route of administration is the option of choice but has only been achieved so far by CRBN- recruiting bifunctional degrader molecules. We aimed to achieve orally bioavailable molecules that selectively degrade the BAF Chromatin Remodelling complex ATPase SMARCA2 over its closely related paralogue SMARCA4, to allow in vivo evaluation of the synthetic lethality concept of SMARCA2 dependency in SMARCA4-deficient cancers. Here we outline structure- and property-guided approaches that led to orally bioavailable VHL-recruiting degraders. Our tool compound, ACBI2, shows selective degradation of SMARCA2 over SMARCA4 in ex vivo human whole blood assays and in vivo efficacy in SMARCA4-deficient cancer models. This study demonstrates the feasibility for broadening the E3 ligase and physicochemical space that can be utilised for achieving oral efficacy with bifunctional molecules.

Protein degraders are an emerging drug modality; however, their properties lie beyond typical drug-like space. Here the authors report optimisation via structure-based exit vector and linker design towards the VHL-recruiting PROTAC ACBI2, an orally bioavailable and selective degrader of SMARCA2.

Abstract 3849: Structure-based PROTAC design demonstrates BAF complex ATPase vulnerabilities in cancer

Targeting subunits of BAF/PBAF complexes has been proposed as an approach to exploit cancer vulnerabilities, yet small molecules developed to date have remained largely ineffectual. Here we develop PROTAC degraders of the BAF ATPase subunits SMARCA2 and SMARCA4 using a bromodomain ligand and recruitment of the E3 ubiquitin ligase VHL. High-resolution ternary complex crystal structures and biophysical investigation guided the rational optimization of ACBI1, a potent cooperative SMARCA2/4 degrader. ACBI1 induced antiproliferative effects and cell death caused by SMARCA2 depletion in SMARCA4 mutant cancer cells, and in acute myeloid leukemia cells dependent on SMARCA4 ATPase activity. These findings exemplify a successful biophysics and structure driven campaign to degrade targets previously considered undruggable, and pave the way towards new therapeutics for the treatment of tumors sensitive to the loss of BAF complex ATPases.

Citation Format: Manfred Koegl, William Farnaby, Claire Whitworth, Michael Roy, Emelyne Diers, Nicole Trainor, Steffen Steurer, Carina Riedmueller, Teresa Gmaschitz, Johannes Wachter, Christian Dank, Michael Gallant, Bernadette Sharps, Klaus Rumpel, Elisabeth Traxler, Romario Lorenz, Oliver Petermann, Peter Greb, Harald Weinstabl, Gerd Bader, Andreas Zoephel, Alexander Weiss-Puxbaum, Joerg Rinnenthal, Heribert Arnnhof, Nicola Wiechens, Meng-Ying Wu, Tom Owen-Hughes, Peter Ettmayer, Mark Pearson, Darryl B. McConnell, Alessio Ciulli. Structure-based PROTAC design demonstrates BAF complex ATPase vulnerabilities in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3849.

BAF complex vulnerabilities in cancer demonstrated via structure-based PROTAC design

Targeting subunits of BAF/PBAF chromatin remodeling complexes has been proposed as an approach to exploit cancer vulnerabilities. Here we develop PROTAC degraders of the BAF ATPase subunits SMARCA2 and SMARCA4 using a bromodomain ligand and recruitment of the E3 ubiquitin ligase VHL. High-resolution ternary complex crystal structures and biophysical investigation guided rational and efficient optimization towards ACBI1, a potent and cooperative degrader of SMARCA2, SMARCA4 and PBRM1. ACBI1 induced antiproliferative effects and cell death caused by SMARCA2 depletion in SMARCA4 mutant cancer cells, and in acute myeloid leukemia cells dependent on SMARCA4 ATPase activity. These findings exemplify a successful biophysics- and structure-based PROTAC design approach to degrade high profile drug targets and pave the way towards new therapeutics for the treatment of tumors sensitive to the loss of BAF complex ATPases.

SPR-Measured Dissociation Kinetics of PROTAC Ternary Complexes Influence Target Degradation Rate

Bifunctional degrader molecules, known as proteolysis-targeting chimeras (PROTACs), function by recruiting a target to an E3 ligase, forming a target/PROTAC/ligase ternary complex. Despite the importance of this key intermediate species, no detailed validation of a method to directly determine binding parameters for ternary complex kinetics has been reported, and it remains to be addressed whether tuning the kinetics of PROTAC ternary complexes may be an effective strategy to improve the efficiency of targeted protein degradation. Here, we develop an SPR-based assay to quantify the stability of PROTAC-induced ternary complexes by measuring for the first time the kinetics of their formation and dissociation in vitro using purified proteins. We benchmark our assay using four PROTACs that target the bromodomains (BDs) of bromodomain and extraterminal domain proteins Brd2, Brd3, and Brd4 to the von Hippel-Lindau E3 ligase (VHL). We reveal marked differences in ternary complex off-rates for different PROTACs that exhibit either positive or negative cooperativity for ternary complex formation relative to binary binding. The positively cooperative degrader MZ1 forms comparatively stable and long-lived ternary complexes with either Brd4BD2 or Brd2BD2 and VHL. Equivalent complexes with Brd3BD2 are destabilized due to a single amino acid difference (Glu/Gly swap) present in the bromodomain. We observe that this difference in ternary complex dissociative half-life correlates to a greater initial rate of intracellular degradation of Brd2 and Brd4 relative to Brd3. These findings establish a novel assay to measure the kinetics of PROTAC ternary complexes and elucidate the important kinetic parameters that drive effective target degradation.

Fatal Calcific Uraemic Arteriolopathy (CUA): A Case Report and Review of the Literature

Calciphylaxis, now better known as Calcific uraemic arteriolopathy (CUA), is an uncommon condition characterised by small vessel calcification and occlusion with resultant painful violaceous skin lesions that typically ulcerate to form non-healing gangrenous ulcers. The syndrome is usually found in patients with renal failure. In this report we describe a 61 year old lady who developed lower limb ulceration secondary to calciphylaxis and discuss the current treatment options for this serious condition.

A microRNA allele that emerged prior to apple domestication may underlie fruit size evolution

The molecular genetic mechanisms underlying fruit size remain poorly understood in perennial crops, despite size being an important agronomic trait. Here we show that the expression level of a microRNA gene (miRNA172) influences fruit size in apple. A transposon insertional allele of miRNA172 showing reduced expression associates with large fruit in an apple breeding population, whereas over-expression of miRNA172 in transgenic apple significantly reduces fruit size. The transposon insertional allele was found to be co-located with a major fruit size quantitative trait locus, fixed in cultivated apples and their wild progenitor species with relatively large fruit. This finding supports the view that the selection for large size in apple fruit was initiated prior to apple domestication, likely by large mammals, before being subsequently strengthened by humans, and also helps to explain why signatures of genetic bottlenecks and selective sweeps are normally weaker in perennial crops than in annual crops.

The impact of brief tobacco treatment training on practice behaviours, self-efficacy and attitudes among healthcare providers

INTRODUCTION AND PURPOSE

The need for aggressive efforts to help tobacco users quit remains a healthcare priority. Brief interventions delivered in the healthcare environment continue to be a valuable component of a comprehensive tobacco control policy. Unfortunately, such treatments are offered less often than desired, and quality is variable. Previous research has demonstrated the value of training experiences in increasing treatment availability, quality, as well as improving clinical outcomes. Less is known about how specific clinical activities and other features are impacted as a function of training. These issues were explored within the context of a standardised 5A's (ASK, ADVISE, ASSESS, ASSIST, ARRANGE) brief intervention training program.

METHODS

A variety of healthcare providers participated in this study. Survey methodology was employed to collect Practice Behaviour, Self-Efficacy and Attitude ratings at pretraining, post training and 6-month follow-up. Linear mixed effects models were used to evaluate primary outcomes, and linear regression to explore the relationships among clinical variables.

RESULTS

Pretraining data suggested overall modest levels of tobacco treatment activity, with limited direct intervention (ASSIST) or follow-up (ARRANGE) efforts. The training experience was shown to have a substantial and sustained impact on 5A's Practice Behaviour ratings, and other clinical indicators (all Pre vs. Post and Pre vs. Follow-up comparisons p < 0.001). Self-Efficacy at post training predicted practice behaviours at follow-up (for ADVISE, ASSESS, ASSIST and ARRANGE: all p's < 0.05).

CONCLUSIONS

The value of a structured training experience was confirmed, and findings served to clarify the specific nature of training program impact.

Cell survival signalling through PPARδ and arachidonic acid metabolites in neuroblastoma

Retinoic acid (RA) has paradoxical effects on cancer cells: promoting cell death, differentiation and cell cycle arrest, or cell survival and proliferation. Arachidonic acid (AA) release occurs in response to RA treatment and, therefore, AA and its downstream metabolites may be involved in cell survival signalling. To test this, we inhibited phospholipase A2-mediated AA release, cyclooxygenases and lipoxygenases with small-molecule inhibitors to determine if this would sensitise cells to cell death after RA treatment. The data suggest that, in response to RA, phospholipase A2-mediated release of AA and subsequent metabolism by lipoxygenases is important for cell survival. Evidence from gene expression reporter assays and PPARδ knockdown suggests that lipoxygenase metabolites activate PPARδ. The involvement of PPARδ in cell survival is supported by results of experiments with the PPARδ inhibitor GSK0660 and siRNA-mediated knockdown. Quantitative reverse transcriptase PCR studies demonstrated that inhibition of 5-lipoxygenase after RA treatment resulted in a strong up-regulation of mRNA for PPARδ2, a putative inhibitory PPARδ isoform. Over-expression of PPARδ2 using a tetracycline-inducible system in neuroblastoma cells reduced proliferation and induced cell death. These data provide evidence linking lipoxygenases and PPARδ in a cell survival-signalling mechanism and suggest new drug-development targets for malignant and hyper-proliferative diseases.

Genomic selection for fruit quality traits in apple (Malus×domestica Borkh.)

The genome sequence of apple (Malus×domestica Borkh.) was published more than a year ago, which helped develop an 8K SNP chip to assist in implementing genomic selection (GS). In apple breeding programmes, GS can be used to obtain genomic breeding values (GEBV) for choosing next-generation parents or selections for further testing as potential commercial cultivars at a very early stage. Thus GS has the potential to accelerate breeding efficiency significantly because of decreased generation interval or increased selection intensity. We evaluated the accuracy of GS in a population of 1120 seedlings generated from a factorial mating design of four females and two male parents. All seedlings were genotyped using an Illumina Infinium chip comprising 8,000 single nucleotide polymorphisms (SNPs), and were phenotyped for various fruit quality traits. Random-regression best liner unbiased prediction (RR-BLUP) and the Bayesian LASSO method were used to obtain GEBV, and compared using a cross-validation approach for their accuracy to predict unobserved BLUP-BV. Accuracies were very similar for both methods, varying from 0.70 to 0.90 for various fruit quality traits. The selection response per unit time using GS compared with the traditional BLUP-based selection were very high (>100%) especially for low-heritability traits. Genome-wide average estimated linkage disequilibrium (LD) between adjacent SNPs was 0.32, with a relatively slow decay of LD in the long range (r² = 0.33 and 0.19 at 100 kb and 1,000 kb respectively), contributing to the higher accuracy of GS. Distribution of estimated SNP effects revealed involvement of large effect genes with likely pleiotropic effects. These results demonstrated that genomic selection is a credible alternative to conventional selection for fruit quality traits.

Time response of carboplatin-induced hearing loss in rat

Carboplatin is currently being used as an anticancer drug against human cancers. However, high dose of carboplatin chemotherapy resulted in hearing loss in cancer patients. We have shown that carboplatin-induced hearing loss was related to dose-dependent oxidative injury to the cochlea in rat model. However, the time response of ototoxic dose of carboplatin on hearing loss and oxidative injury to cochlea has not been explored. The aim of the study was to evaluate the time response of carboplatin-induced hearing loss and oxidative injury to the cochlea of the rat. Male Wistar rats were divided into two groups of 30 animals each and treated as follows: (1) control (normal saline, i.p.) and (2) carboplatin (256 mg/kg, a single i.p. bolus injection). Auditory brain-evoked responses (ABRs) were recorded before and 1-5 days after treatments. The animals (n = 6) from each group were sacrificed on day 1, 2, 3, 4, and 5 and cochleae were isolated and analyzed. Carboplatin significantly elevated the hearing thresholds to clicks and to 2, 4, 8, 16, and 32 kHz tone burst stimuli only 3-5 days post-treatment. Carboplatin significantly increased nitric oxide (NO), malondialdehyde (MDA) levels and manganese superoxide dismutase (Mn-SOD) activity in the cochlea 4-5 and 3-5 days post-treatment, respectively, indicating enhanced influx of free radicals and oxidative injury to the cochlea. Carboplatin significantly depressed the reduced to oxidized glutathione (GSH/GSSG) ratio, antioxidant enzyme activities such as copper/zinc-superoxide dismutase (CuZn-SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) as well as enzyme protein expressions in the cochlea 3-5 days after treatment. The data suggest that carboplatin-induced hearing loss involves oxidative injury to the cochlea of the rat in a time-dependent manner.

Carboplatin-induced oxidative stress in rat cochlea

Carboplatin is currently being used in the clinic against a variety of human cancers. However, high dose carboplatin chemotherapy resulted in ototoxicity in cancer patients. This is the first study to show carboplatin-induced oxidative stress response in the cochlea of rat. Male Wistar rats were divided into two groups of six animals each and treated as follows: (1) control (normal saline, i.p.) and (2) carboplatin (256 mg/kg, i.p.). Animals in both groups were sedated with ketamine/xylazine and auditory brainstem-evoked responses were recorded before and 4 days after treatments. The animals were sacrificed on the fourth day and cochleae were harvested and analyzed. A significant elevation of the hearing threshold shifts was noted at clicks, 8, 16, and 32 kHz tone burst stimuli following carboplatin administration. Carboplatin significantly increased nitric oxide and malondialdehyde levels, xanthine oxidase and manganese-superoxide dismutase activities in the cochlea indicating enhanced flux of free radicals. Cochlear glutathione levels, antioxidant enzyme activities such as copper zinc-superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glutathione S-transferase and enzyme protein levels were significantly depleted 4 days after carboplatin treatment. The data suggest that carboplatin induced free radical generation and antioxidant depletion, and caused oxidative injury in the cochleae of rats.

Dose response of carboplatin-induced hearing loss in rats: antioxidant defense system

Carboplatin, a platinum-containing anticancer drug, is currently being used against a variety of cancers. However, a single high dose of carboplatin is ototoxic in cancer patients. This is the first study to show carboplatin-induced hearing loss in a rat model. Male Wistar rats were divided into five groups and treated as follows: (1) control (saline, intraperitoneally (i.p.)); (2) carboplatin (64 mg/kg, i.p.); (3) carboplatin (128 mg/kg i.p.); (4) carboplatin (192 mg/kg, i.p.) and (5) carboplatin (256 mg/kg, i.p.). Animals in all groups were sedated with ketamine/xylazine and auditory brain-evoked responses (ABRs) were recorded before and 4 days after treatments. The animals were sacrificed on the fourth day and cochleae were harvested and analyzed. Carboplatin dose-dependently decreased body weight. However, at higher doses of carboplatin (192 and 256 mg/kg), there was a significant elevation of hearing threshold shifts at clicks, 4, 8, 16 and 32 kHz tone burst stimuli. The higher doses of carboplatin (192 and 256 mg/kg) significantly increased cochlear lipid peroxidation (132 and 146% of control) and depleted cochlear glutathione levels (66 and 63% of control), respectively. The antioxidant enzyme activities such as superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, and glutathione-S-transferase (GST) depressed significantly at higher doses of carboplatin. The data suggest that higher doses of carboplatin (above 128 mg/kg) induce hearing loss as evidenced by significant changes in ABRs, lipid peroxidation and antioxidants in the cochlea of rats.

Effect of protective agents against cisplatin ototoxicity

HYPOTHESIS

The goals of this investigation were to compare the efficacy of three protective agents against cisplatin-induced elevation of auditory brainstem response (ABR) thresholds and to examine whether these protective agents prevent cisplatin-induced alterations of the antioxidant defense system in the cochlea of the rat.

BACKGROUND

Cisplatin is an ototoxic antitumor agent. Previous animal studies have shown that cisplatin administration causes an elevation of ABR thresholds. These auditory changes are accompanied by alterations in the concentration of glutathione and the antioxidant enzymes in the cochlea. The authors' previous work has indicated that the protective agent diethyldithiocarbamate (DDTC) prevents decrease in glutathione (GSH), alteration of antioxidant enzyme activity, and disruption of cochlear function with cisplatin administration.

METHODS

Wistar rats were sedated and underwent pretreatment ABR testing using clicks and tone burst stimuli at 8, 16, and 32 kHz. Control rats received saline by intraperitoneal (i.p.) injection. Positive control rats were administered cisplatin 16 mg/kg i.p. Three groups of rats received protective agents in combination with cisplatin. The DDTC-protected rats were given 600 mg/kg of DDTC subcutaneously 1 hour after cisplatin. Animals protected by 4-methylthiobenzoic acid (MTBA) were given 250 mg/kg of this agent i.p. 30 minutes before cisplatin. Animals protected with ebselen were given 16 mg/kg i.p. one hour before cisplatin. The ABR thresholds were recorded 72 hours after cisplatin administration in all groups. Cochleas were removed, and extracts of the tissues were analyzed for GSH, activities of antioxidant enzymes (superoxide dismutase [SOD], catalase, glutathione peroxidase, and glutathione reductase) and malondialdehyde (MDA) (as an index of lipid peroxidation).

RESULTS

Cisplatin-treated rats had significant ABR threshold shifts, ranging from 27 to 40 dB. Rats administered each of the three protective agents in combination with cisplatin had ABR threshold shifts of <10 dB. The cochleae of rats administered cisplatin alone had nearly a 50% depletion of glutathione and about a 50% reduction in the activities of SOD, glutathione peroxidase, and glutathione reductase, while catalase activity was reduced to 70% of control values. These changes were accompanied by a reciprocal elevation of MDA of 165%. These changes, namely, the depletion of GSH and antioxidant enzyme activity and the elevation of MDA in the cochlea, were largely attenuated by the administration of the protective agents tested.

CONCLUSION

These findings suggest that cisplatin ototoxicity is related to lipid peroxidation and that the use of protective agents prevents hearing loss and lipid peroxidation by sparing the antioxidant system in the cochlea. These results suggest the possibility that the clinical use of protective agents could effectively reduce or prevent damage to the inner ear of patients receiving cisplatin chemotherapy, provided that the antitumor effect is not altered.

Dose-dependent protection by lipoic acid against cisplatin-induced nephrotoxicity in rats: antioxidant defense system

This study was designed to investigate the role of graded doses of lipoic acid pretreatment against cisplatin-induced nephrotoxicity. Male Wistar rats were divided into six groups and treated as follows: 1) vehicle (saline) control; 2) cisplatin (16 mg/kg, intraperitoneally); 3) lipoic acid (100 mg/kg, intraperitoneally); 4) cisplatin plus lipoic acid (25 mg/kg); 5) cisplatin plus lipoic acid (50 mg/kg) and 6) cisplatin plus lipoic acid (100 mg/kg). Rats were sacrificed three days after treatment, and plasma as well as kidneys were isolated and analyzed. Plasma creatinine increased (677% of control) following cisplatin administration alone which was decreased by lipoic acid in a dose-dependent manner. Cisplatin-treated rats showed a depletion of renal glutathione (GSH), increased oxidized GSH and decreased GSH/GSH oxidized ratio (62%, 166% and 62% of control), respectively which were restored with lipoic acid pretreatment. Renal superoxide dismutase, catalase, glutathione peroxidase (GSH peroxidase) and glutathione reductase activities decreased (62%, 75%, 62% and 80% of control), respectively, and malondialdehyde content increased (204% of control) following cisplatin administration, which were restored with increasing doses of lipoic acid. The renal platinum concentration increased following cisplatin administration, which was possibly decreased by chelation with lipoic acid. The data suggest that the graded doses of lipoic acid effectively prevented a decrease in renal antioxidant defense system and prevented an increase in lipid peroxidation, platinum content and plasma creatinine concentrations in a dose-dependent manner.

Effect of cisplatin on the expression of glutathione-S-transferase in the cochlea of the rat

Glutathione-S-transferase (GST) has been found to conjugate glutathione (GSH) to diverse electrophiles and play a major role in the detoxification of alkylating and platinating agents. However, there is little information regarding the pattern of GST expression in the cochlea. Although cisplatin is ototoxic, its effect on GST in the cochlea is unknown. In the present study we investigated the pattern of GST expression in control and cisplatin-treated cochleas by using the laboratory rat as animal model. Sixteen mature rats were randomly assigned to control or cisplatin groups. After treatment, cochleas were procured and tissues stained by immunohistochemical methods to detect the presence of GST. Optical density measurements were determined to gauge intensity of GST immunostaining. Strong positive GST immunostaining was demonstrated in all cochleas, with the most intense staining found in the spiral ligament and the least in Reissner's membrane. Mean optic density scores were lower for cisplatin-treated cochleas than for control cochleas in all areas analyzed. These findings showed that GST is expressed throughout the rat cochlea, with cisplatin treatment causing its decreased expression.

Application of antioxidants and other agents to prevent cisplatin ototoxicity

OBJECTIVE/HYPOTHESIS

To review the recent data from experiments performed in this laboratory to test the hypothesis that cisplatin ototoxicity is related to depletion of glutathione and antioxidant enzymes in the cochlea and that the use of antioxidants or protective agents would protect the cochlea against cisplatin damage and prevent hearing loss.

STUDY DESIGN/METHODS

Data were reviewed from experiments performed in this laboratory. Control rats were treated intraperitoneally with cisplatin 16 mg/kg. Experimental rats were given cisplatin in combination with one of the following protective agents: diethyldithiocarbamate, 4-methylthiobenzoic acid, ebselen, or lipoic acid. Animals in each group underwent auditory brainstem response (ABR) threshold testing before and 3 days after treatment. Cochleae were removed after final ABR testing and analyzed for glutathione and activities of the enzymes superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, and malondialdehyde.

RESULTS

Rats in the control group receiving cisplatin were found to have significant ABR threshold shifts. This was accompanied by a reduction of glutathione and the activity of antioxidant enzymes (superoxide dismutase, glutathione peroxidase, catalase, and glutathione reductase) and an elevation of malondialdehyde. Experimental animals had preservation of ABR thresholds and levels of glutathione, antioxidant enzyme activity, and malondialdehyde that were similar to untreated animals.

CONCLUSION

Cisplatin ototoxicity appears to be initiated by fee-radical production, which causes depletion of glutathione and antioxidant enzymes in the cochlea, and lipid peroxidation, manifested by an increase in malondialdehyde. These effects were blocked by each of a series of antioxidant compounds given in combination with cisplatin. A mechanism for cisplatin ototoxicity is elaborated with a proposed plan of chemoprevention using agents with different mechanisms of action. These substances could be used alone or in combination to reduce the severity of cisplatin ototoxicity in patients.

Dose dependent protection by lipoic acid against cisplatin-induced ototoxicity in rats: antioxidant defense system

This study investigated the alterations that occur in auditory brainstem-evoked responses (ABRs) concurrent with changes in cochlear concentrations of glutathione (GSH), lipid peroxidation, and antioxidant enzyme activity in cisplatin-induced ototoxicity and in dose-dependent otoprotection by an antioxidant lipoate. Male Wistar rats were divided into different groups and were treated as follows, with: (1) vehicle (saline) control; (2) cisplatin (16 mg/kg, i.p.); (3) lipoate (100 mg/kg, i.p.) plus saline; (4) cisplatin plus lipoate (25 mg/kg); (5) cisplatin plus lipoate (50 mg/kg), and (6) cisplatin plus lipoate (100 mg/kg). Post-treatment ABRs were evaluated after three days, the rats were sacrificed, and cochleae were harvested and analyzed. The cisplatin-injected rats showed ABR threshold elevations above the pre-treatment thresholds. Rats treated with lipoate plus cisplatin did not show significant elevation of hearing thresholds. Cisplatin administration resulted in a depletion of cochlear GSH concentration (69% of control), whereas, cisplatin-plus-lipoate treatment increased GSH concentration close to control value. Cisplatin-treated rats showed a decrease in cochlear superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione reductase (GR) activities (57, 78, 59, and 58% of control, respectively), and an increase in malondialdehyde (MDA) concentration (196% of control). Cochlear SOD, CAT, GSH-Px, and GR activities and MDA concentrations were restored in the rats injected with cisplatin plus graded doses of lipoate than those with cisplatin alone. It is concluded that cisplatin-induced ototoxicity is related to impairment of the cochlear antioxidant defense system, and the dose-dependent otoprotection conferred by an antioxidant lipoate against cisplatin ototoxicity is associated with sparing of the cochlear antioxidant defense system.

Protection by ebselen against cisplatin-induced nephrotoxicity: antioxidant system

This study was designed to investigate the cisplatin-induced alteration in renal antioxidant system and the nephroprotection with ebselen. Male Wistar rats were injected with (1) vehicle control; (2) cisplatin; (3) ebselen; and (4) cisplatin plus ebselen. Rats were sacrificed three days post-treatment and plasma as well as kidney were isolated and analyzed. Plasma creatinine increased 598% following cisplatin administration alone which decreased by 158% with ebselen pretreatment. Cisplatin-treated rats showed a depletion of renal glutathione (GSH) levels (52% of control), while cisplatin plus ebselen injected rats had GSH values close to the controls. Antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities decreased 38, 75 and 62% of control, respectively, and malondialdehyde (MDA) levels increased 174% of control following cisplatin administration, which were restored to control levels after ebselen treatment. The renal platinum level did not significantly change with ebselen pretreatment. This study suggests that the protection offered by ebselen against cisplatin-induced nephrotoxicity is partly related to the sparing of antioxidant system.

Quantification of furosemide from serum and tissues using high-performance liquid chromatography

Since pharmacokinetics may play a significant role in furosemide (FSM) developmental ototoxicity, we developed an assay for the extraction and quantification of FSM in tissue and fluid from neonatal and adult rats. Rats from post-natal day (PND) 10, 30 and 50, were given an intravenous dose of FSM (35 mg/kg). Blood and tissues were analyzed by HPLC. FSM in serum, perilymph and liver was elevated in PND ten rats as was the body burden of FSM. Renal concentrations were higher in older rats. Altered clearance of FSM in developing rats may result in higher concentrations in the cochlea and ototoxicity.

Protection by 4-methylthiobenzoic acid against cisplatin-induced ototoxicity: antioxidant system

This study was undertaken in order to determine the changes in auditory brainstem-evoked responses relationship with the changes in the levels of GSH, lipid peroxidation and antioxidant enzymes activity in cisplatin-induced ototoxicity and otoprotection by 4-methylthiobenzoic acid (MTBA). Male Wistar rats in different groups were treated as follows: 1) saline control; 2) cisplatin (16 mg/kg, intraperitoneally); 3) MTBA (250 mg/kg, intraperitoneally), and 4) cisplatin plus MTBA. Post-treatment auditory brainstem-evoked responses were performed after three days and the rats were sacrificed and cochleae harvested. The cochleae were analyzed for glutathione (GSH), antioxidant enzyme activity, and malondialdehyde levels. The cisplatin injected rats showed a threshold elevation of 31.9 +/- 16.0 dB above the pretreatment thresholds using click stimulus. Rats treated with MTBA plus cisplatin did not show significant elevation of hearing threshold. Cisplatin plus MTBA administration showed a higher levels of cochlear GSH (5.59 +/- 0.35 nmoles/mg protein) compared to cisplatin alone (4.46 +/- 0.13 nmoles/mg protein). Cisplatin treated rats showed a decrease in superoxide dismutase, catalase, glutathione peroxidase (GSH-peroxidase), and glutathione reductase (GSH-reductase) activities (57%, 83%, 78% and 58% of control). Cochlear superoxide dismutase, catalase and GSH-reductase activities and MDA levels were restored in the rats injected with cisplatin plus MTBA, compared to cisplatin alone. It is concluded that the protection conferred by MTBA against cisplatin ototoxicity is associated with sparing of the cochlear antioxidant system.

Up-regulation of adenosine receptors in the cochlea by cisplatin

In a previous study, we have demonstrated the presence of two adenosine receptor (AR) subtypes, namely A1 and A3AR, in the chinchilla cochlea. One or both of these receptors couple to activation of antioxidant enzymes, with resulting decreases in lipid peroxidation. The chemotherapeutic agent, cisplatin, was shown to produce ototoxicity within a few days of administration presumably by generating reactive oxygen species (ROS) and thereby increasing lipid peroxidation. In this study, we focused on whether lipid peroxidation induces hearing loss by assessing the cochlear antioxidant defense system over a shorter time period (24 h) following cisplatin administration. Cisplatin was administered to anesthetized chinchillas by round window membrane application and hearing loss was determined by compound action potential (CAP) and endocochlear potential (EP) 24 and 72 h post-treatment. Elevations in CAP thresholds in response to click and to 2, 4, 8 and 16 kHz tones and decreases in EP were obtained within 24 h of cisplatin treatment. These changes persisted for at least up to 72 h. Measurements of antioxidant enzymes indicate no change in the activities of superoxide dismutase, catalase or glutathione peroxidase, either 24 or 72 h following cisplatin treatment. The levels of malondialdehyde obtained at these time points were equivalent to those obtained from the controls. Furthermore, no difference in cochlear morphology was detectable by scanning electron microscopy at the basal, middle or apical turns of the cochlea within 24 h. By 72 h, however, losses in both inner and outer hair cells were observed in the basal and middle turns of the cochlea. A major finding of this study is that exposure to cisplatin led to a 5-fold up-regulation of [125I]N6-2-[4-amino-3-phenyl]ethyladenosine binding in the cochlea within 24 h, reflecting increases in expression of AR(s) in this tissue. These data indicate a dissociation between cisplatin acute (within 24 h) ototoxicity and lipid peroxidation. Furthermore, up-regulation of AR(s) may represent a rapid compensatory mechanism by the cochlea to counter the toxic effects of increased ROS generated by cisplatin.

Expression and function of adenosine receptors in the chinchilla cochlea

Previous studies indicate the presence of adenosine receptors in the cochlear tissues obtained from different animals. This study was initiated to determine the subtypes of adenosine receptor (AR) present in the chinchilla cochlea and to assess their function. Radioligand binding studies demonstrate the presence of both the A1AR and A3AR in membranes prepared from the cochlea, using the radioligands [3H]DPCPX and [125I]APNEA. Estimates of the number (Bmax) of A1AR and A1AR plus A3AR by saturation curves were 118 +/- 13 and 417 +/- 120 fmol/mg, respectively, with the respective equilibrium dissociation constants (Kd) averaging 2.7 +/- 0.2 and 26.3 +/- 13.8 nM. No significant number of A2aAR were detected using [3H]CGS21680. The nonhydrolyzable adenosine analog R-phenylisopropyladenosine (R-PIA, 1 microM) elicited a small but significant degree of inhibition of forskolin-stimulated adenylyl cyclase activity (10.4 +/- 2.5%) in cochlear membrane preparations, which was insensitive to blockade by theophylline (100 microM). Furthermore, R-PIA elicited an increase in inositol 1,4,5-trisphosphate production in dissociated cell preparations obtained from the cochlea. No significant effect of R-PIA was observed on auditory measures such as auditory brainstem evoked response, cochlear action potential and endocochlear potential following round window application. However, round window application of R-PIA elicited significant increases in the activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase and significantly reduced the levels of malondialdehyde, a marker of lipid peroxidation. These results suggest a potential cytoprotective role of adenosine in the cochlea against oxidative damage.

4-methylthiobenzoic acid protection against cisplatin nephrotoxicity: antioxidant system

This study investigates the changes in renal antioxidant system after cisplatin administration and the nephroprotection with 4-methylthiobenzoic acid (MTBA). Male Wistar rats were injected with (1) vehicle control, (2) cisplatin, (3) MTBA, and (4) cisplatin plus MTBA. Rats were euthenized 3 days post-treatment and kidney was isolated and analyzed for platinum concentration, malondialdehyde (MDA), glutathione (GSH and GSSG), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px). Plasma creatinine increased 508% following cisplatin administration alone, which decreased to 189% with MTBA. Cisplatin-treated rats showed a depletion of renal GSH levels (53%), while cisplatin plus MTBA-injected rats had GSH values close to those of the controls. SOD, CAT, and GSH-Px activities decreased 36, 29, and 38%, respectively, and MDA levels increased 212% following cisplatin administration, which were restored to control levels after MTBA treatment. The renal platinum level depleted significantly with MTBA treatment. The data suggest that cisplatin nephrotoxicity is mediated by depletion in GSH concentration and by impaired activities of SOD, CAT, and GSH-Px, increased lipid peroxidation, and plasma creatinine levels. The protection offered by MTBA against cisplatin nephrotoxicity is related to the reduction in plasma creatinine levels, prevention of GSH depletion and lipid peroxidation, and restoring antioxidant enzyme activity in the kidneys of rats.

Combined effects of adrenalectomy and noise exposure on compound action potentials, endocochlear potentials and endolymphatic potassium concentrations

The effects of removal of endogenous corticosteroids via bilateral adrenalectomy in combination with noise exposure (30 min at 100 dB) were determined by recording compound action potential (CAP) and endocochlear potentials (EP), and by measuring potassium concentrations (K+e) within the endolymph. Thirty-eight Long-Evans rats were divided into groups according to experimental treatments: adrenalectomy (ADX) or non-ADX and noise exposure or non-noise exposure. CAP thresholds, EP and K+e values were subjected to repeated-measures analysis of variance with group and time as factors classifying the measurements. Noise exposure resulted in significant elevations of CAP thresholds in both the ADX and non-ADX animals, but had no effect on either EP or endolymphatic K+e. Recovery was noted during all post-exposure measurement periods and was significantly faster for ADX animals. EP and K+e did not change during or after noise exposure. ADX animals showed a non-significant reduction of EP and a statistically significant increase of K+e during all measurement periods as compared to non-ADX animals.

Cochlear effects of indacrinone are not altered by penicillin

Indacrinone is a loop diuretic structurally related to ethacrynic acid. Indacrinone is a racemic mixture. Previous studies have shown that the (-) enantiomer caused reduction of endocochlear potential (EP) and elevation of compound action potential (CAP) threshold (Rybak and Whitworth, 1987a). It has been demonstrated that organic acids such as penicillin, probenecid and sodium salicylate prevent the reduction of EP normally observed after furosemide administration (Rybak et al., 1992a). The present study was designed to determine whether penicillin pretreatment could prevent changes in EP and CAP threshold in (-)-indacrinone treated chinchillas. Adult chinchillas were anesthetized with ketamine and pentobarbital. A microelectrode was advanced into the scala media using the round window approach, and CAP responses to clicks were measured. One group was treated with (-)-indacrinone 100 mg/kg via the jugular vein. A second group of animals received penicillin 50 mg/kg i.v. thirty minutes before (-)-indacrinone. The mean EP change in the indacrinone-treated animals was 38.38 +/- 19.32 millivolts (mv). The reduction of EP in the group receiving penicillin was 24.43 +/- 20.74 mv (P > 0.09). The mean CAP threshold changes in animals receiving indacrinone was 20 +/- 14.14 dB whereas those pretreated with penicillin showed a threshold shift of 21.43 +/- 20.35 dB (P > 0.05). These findings are consistent with previous studies which showed that the effect of ethacrynic acid on the EP and CAP was not changed by the pretreatment with penicillin (Rybak et al., 1990).

Structure-activity studies on 2-aryl-4H-3,1-benzoxazin-4-ones

Eight benzoxazin-4-ones related in structure to NSC 341964 (1) have been tested for cytotoxicity in two different cell systems. Two of the benzoxazin-4-ones (3 and 10) showed good cytotoxicity (ID50 = 9.9 and 8.9 microM) in P388 cells. The nitrobenzoxazin-4-one (10) caused a significant alteration in cell cycle distribution when administered to P388 cells and was an inhibitor of porcine pancreatic elastase. Structure-activity relationships are discussed.

Identification of A1 adenosine receptors in rat cochlea coupled to inhibition of adenylyl cyclase

A1 adenosine receptors (A1ARs) are found in a number of tissues in the body where their physiological roles have been identified. In the cochlea, neither the existence of these receptors nor a physiological role of adenosine has been described previously. Membranes prepared from rat cochlea demonstrated high affinity and saturable binding to N6-2-(4-amino-3-[125I]iodophenyl)ethyladenosine ([125I]APNEA), an A1AR agonist, with maximum binding capacity and dissociation constant values being 40.5 +/- 0.5 fmol/mg protein and 1.28 +/- 0.03 nM, respectively. Adenosine analogues competed for [125I]APNEA binding sites with a rank order of potency characteristic of these sites being the A1AR. The [125I]APNEA binding was significantly reduced by pertussis toxin, indicating coupling of these receptors with the Gi and/or Go proteins in cochlear membranes. Photoaffinity labeling of the receptor protein with the A1AR agonist N6-2-(4-azido-3[125I]iodophenyl)ethyladenosine showed specific labeling of a 36-kDa receptor protein. Activation of the A1AR with R-phenylisopropyladenosine (R-PIA) led to inhibition of forskolin-stimulated adenylyl cyclase activity. Amplification of reverse-transcribed RNA derived from cochlear tissue by polymerase chain reaction (using primers for the bovine A1AR) yielded a 770-bp product that hybridized to an A1AR cDNA probe on Southern blots. These data indicate the presence of an inhibitory receptor in the peripheral auditory system, which may play an important role in modulating auditory functions.

Kinocilia in the developing stria vascularis of the rat pup

The mammalian stria vascularis undergoes certain developmental changes in the postnatal rat. The present study was designed to examine the ultrastructure of the stria vascularis in rat pups from immediately after birth to 20 days postpartum. The cochlea were removed with the animals under xylazine (Rompun) anesthesia and were prepared for transmission electron microscopy. Each of the three cell types in the stria were found to contain kinocilia up until 12-17 days of age. The presence of kinocilia in the intermediate and basal cells has not been previously described. Findings suggest that these organelles may serve a motile and/or sensory function to assist in the maturation of cell functions, particularly ion transport, during early stages of development.

Dose-response relationships for furosemide ototoxicity in rat

Furosemide is an ototoxic loop diuretic which is highly bound to serum albumin. Previous studies have shown that rats deficient in albumin are more susceptible to furosemide ototoxicity than are rats with normal serum albumin concentrations. The present study was designed to compare the dose-response relationships for furosemide ototoxicity in rats with normal serum albumin concentration to rats without albumin in their serum. Young adult rats 50-80 days of age from each group were anesthetized with Rompun, and the endocochlear potential (EP) and compound action potential (CAP) thresholds were measured before and after furosemide injection. Afer a stable EP and CAP threshold were measured, each animal was injected with a single dose of furosemide through a cannula in the jugular vein. Rats with normal serum albumin had very little change in the EP or CAP threshold until the dose of furosemide was 40 mg/kg or greater. The dose-response curves for EP reduction and CAP threshold elevation then rose steeply to reach a maximum at 50 mg/kg. Albumin-deficient rats were much more sensitive to the effects of furosemide. The dose-response curves for both EP and CAP were shifted to the left. The doses resulting in half-maximal effects in the albumin-deficient rats were about half that found in the normal rats. These findings support the hypothesis that the access of furosemide to its site of ototoxic action in the cochlea depends on the quantity of unbound furosemide in the serum.

Furosemide ototoxicity is enhanced in analbuminemic rats

OBJECTIVE

The purpose of this study was to investigate the effect of furosemide on the endocochlear potential (EP) of Sprague-Dawley rats and rats that lack albumin in their serum (Nagase analbuminemic rats [NAR]).

DESIGN

Group comparisons between analbuminemic rats and normal Sprague-Dawley rats was carried out, with statistical evaluation using the Student's t test.

SETTING

Experiments were carried out in a sound-attenuated booth in a research laboratory.

SUBJECTS

Young adult Sprague-Dawley and analbuminemic rats (NAR) 50 to 80 days of age were used as experimental animals.

INTERVENTIONS

Subjects were anesthetized with ketamine and xylazine. Furosemide, 35 mg/kg, was injected intravenously in each of three groups: NAR rats, NAR rats pretreated with albumin and normal Sprague-Dawley rats.

MAIN OUTCOME MEASURES

Endocochlear potential was measured via the round window membrane approach. Urine samples were collected with a metabolic cage, and volumes were recorded.

RESULTS

Sprague-Dawley rats had a very slight EP reduction following furosemide. The NAR rats, however, were found to have an extremely large and rapid reduction of the EP one order of magnitude greater. The NAR rats pretreated with albumin had a significantly smaller reduction of EP than NAR rats not receiving albumin. However, NAR rats pretreated with albumin had a significantly greater urine output than control NAR rats.

CONCLUSIONS

These findings support the hypothesis that the access of furosemide to its site of ototoxic action in the cochlea depends on the quantity of unbound furosemide in the serum.

Effects of organic acids on stria vascularis ultrastructure and function in the chinchilla

The purpose of this study was to compare the effects of several organic acids (probenecid, sodium salicylate and penicillin G) on the endocochlear potential (EP) and the ultrastructure of the stria vascularis of the chinchilla with the effects of furosemide on these parameters. Chinchillas received 50 mg/kg i.v. doses of probenecid, sodium salicylate or penicillin G, or 25 mg/kg i.v. furosemide. The EP was monitored continuously before and for 60 min afterwards. The stria vascularis was removed at 10-min intervals from animals and from 10 to 60 min after the injection of these agents. Specimens were then processed for transmission electron microscopy. Only furosemide had an effect on the EP, causing a reversible reduction. The reduction of the EP was accompanied by the appearance of edema in the intercellular spaces of the stria vascularis. No significant edema was found after probenecid, sodium salicylate or penicillin G. This was consistent with the finding that none of these latter three agents affected the endocochlear potential.

Development of endocochlear potential and compound action potential in the rat

The present study was designed to investigate the developmental changes of the endocochlear potential and compound action potential simultaneously from rat pups of various ages. Animals were anesthetized with ketamine/xylazine, and the endocochlear potential was measured with a glass microelectrode. At the same time, a wire electrode was placed on the round window to record the click-evoked compound action potential. The endocochlear potential was found to be very low during the first few days of postnatal life. A rapid increase in the value of the endocochlear potential was noted between eleven and thirteen days of age, and adult-like values were recorded by seventeen days of age. Compound action potential responses were recorded at thirteen days of age to high intensity clicks, followed by a progressive improvement of thresholds and reduction of latencies. The development of the endocochlear potential and compound action potential was found to be reciprocally related - as the magnitude of the endocochlear potential increased, the compound action potential threshold declined with increasing age. The development of the endocochlear potential was found to closely approximate the development of enzymatic activity of sodium, potassium-ATPase in the stria vascularis reported by Kuijpers (1974).

Effects of organic acids on the edema of the stria vascularis induced by furosemide

Furosemide is a loop diuretic which is ototoxic. Investigations have shown the stria vascularis to be the target tissue of this ototoxic drug. The purpose of the present study was to investigate the effects of furosemide on the stria vascularis in chinchillas, in controls and in animals pretreated with the above organic acids. Control animals were injected with 0.5 ml alkalinized saline followed by furosemide IV 30 min later. Experimental animals received probenecid, penicillin or sodium salicylate IV. Thirty minutes later, furosemide was injected in the same dose as in the controls. The basal turn of the stria vascularis was rapidly removed at various times from 10 to 30 min after furosemide administration and processed for transmission electron microscopy. Control animals were found to have reversible edema of the stria vascularis. Experimental animals had variable findings. Those animals pretreated with penicillin had virtually no edema of the stria vascularis at any time. Salicylate and probenecid pretreated animals had significantly less edema from one to 10 min after furosemide injection, but more edema than controls at later times. These findings suggest a discrepancy between ultrastructural pathology and functional status of the cochlea in experimental animals pretreated with probenecid or sodium salicylate followed by furosemide. On the other hand, good structure function correlations were seen in controls and in experimental animals pretreated with penicillin.

Ototoxicity of furosemide during development

The rat is an altricial animal which serves as a useful model for human auditory development. The purpose of the present study was to investigate the effects of furosemide on cochlear function and the stria vascularis ultrastructure at various postnatal ages. Rat pups anesthetized with ketamine hydrochloride/xylazine hydrochloride received furosemide 35 mg/kg intravenously (IV), and the endocochlear potential and compound action potential of the eighth nerve were recorded. The stria vascularis was removed and prepared for transmission electron microscopy. Rat pups 9 to 28 days of age had a much greater reduction of endocochlear potential and elevation of the compound action potential threshold than animals older than 30 days. These physiologic changes were accompanied by edema of the stria on transmission electron microscopy only in animals at susceptible ages. These findings support the concept of a critical period of susceptibility to ototoxic drugs during development and could have important clinical implications in premature infants.

Comparative acute ototoxicity of loop diuretic compounds

A microelectrode was used to measure endocochlear potentials (EP) in adult chinchillas and to study the effects of a series of loop diuretics. EP was measured before, during and for several hours after the intravenous injection of the following loop diuretics: furosemide, piretanide, bumetanide, ethacrynic acid, indacrinone stereoisomers and ozolinone. The first four loop diuretics caused a substantial dose-related reduction of EP. The (-) isomer of indacrinone was found to cause a dose-related reduction of EP to a moderate degree. The (+) isomer of indacrinone and ozolinone caused very little change of EP, even in very high doses. Findings are consistent with data on the mechanism of action of these agents in the kidney.

Rat as a potential model for hearing loss in biotinidase deficiency

Biotinidase deficiency is an inborn error of metabolism that is transmitted as an autosomal recessive disorder. Symptoms include hearing loss, ataxia, blindness, mental retardation, and seizures. The metabolic defect is an inability to recycle the vitamin biotin, which is an important cofactor in key enzymes. We therefore sought to develop an animal model for this disorder by inducing biotin deficiency. Rat pups were divided into control and experimental groups. Control rats were fed a normal diet whereas experimental animals were given a diet deficient in biotin. Animals from both groups underwent brain stem auditory evoked potential testing at various ages. Wave I thresholds at various ages were similar in both groups. Latencies for wave I, however, and interpeak latencies (I-IV) were prolonged in the biotin-deficient groups compared to controls. Scanning electron microscopy of the organ of Corti in biotin-deficient animals showed no significant hair cell loss. The biotin-deficient developing rat appears to acquire functional changes in the auditory brain stem. These effects may be caused by defective myelination, since biotin is important in fatty acid metabolism.

Development of the stria vascularis in the rat

The rat is an altricial animal and is thus a useful model for the study of auditory development. The endocochlear potential (EP) undergoes a rapid increase in magnitude from the end of the 1st week to the beginning of the 3rd postnatal week. The purpose of this study was to examine the ultrastructure of the developing stria vascularis in the rat pup in order to correlate functional changes with structural alterations. Rat pups of various ages underwent EP measurement under Rompun anesthesia. The cochleas were rapidly removed under deep pentobarbital anesthesia. The tissues were fixed in 2.5% glutaraldehyde and postfixed in 1.5% osmium tetroxide. Thin sections were viewed and photographed using a Hitachi H7000 transmission electron microscope. A series of distinct developmental changes were observed. Intermediate and basal cells became more distinct from one another, and basal cells became more elongated. Marginal cells underwent progressive development of basolateral infoldings. These cytologic changes may signal the development of ion transport mechanisms necessary for EP development.

Organic acids do not alter the cochlear effects of ethacrynic acid

Previous studies have shown a reduction of the ototoxicity of furosemide in chinchillas pretreated with organic acid transport inhibitors. The current studies were designed to investigate whether such a protective effect could be observed in chinchillas receiving ethacrynic acid. Chinchillas weighing 400-600 g were injected with saline followed by ethacrynic acid 12.5 mg/kg i.v. (controls) or one of three organic acids (probenecid, penicillin G and or sodium salicylate) 50 mg/kg i.v., thirty minutes prior to ethacrynic acid injection (experimentals). Endocochlear potential (EP) and compound action potential of the eighth nerve (CAP) elicited by click stimuli were simultaneously monitored before and after injection in both groups. The mean change in EP and CAP findings are consistent with observations by other investigators of the actions of these loop diuretics in the isolated loop of Henle. In the latter tissues, the 'furosemide-like loop diuretics' appear to have a different mechanism of action than does ethacrynic acid. It appears from the findings of the present study that the actions of furosemide and ethacrynic acid on the cochlea are by different mechanisms as well.

Quinine reduces noxious cochlear effects of furosemide and ethacrynic acid

Endocochlear potential (EP) and eighth nerve action potential (AP) were measured in chinchillas. We investigated the interaction of quinine with the loop diuretics furosemide and ethacrynic acid to determine whether the cochlear effects of these agents are attenuated by pretreatment with quinine. Animals were injected with either furosemide, 25 mg/kg intravenously (IV), or ethacrynic acid, 15 mg/kg IV. Control animals injected without pretreatment were found to have a large decrease in EP, with a decrease of compound action potentials (CAP) amplitude and an elevation of CAP threshold. Animals pretreated with quinine, 25 mg/kg, were found to have a significantly smaller reduction of EP and CAP amplitude following injection of either diuretic. No significant differences in urine volumes were noted between experimental and control groups. Quinine is known to cause nonspecific changes in the membranes of epithelial cells, which may cause alterations of the transport of organic anions by such tissues. Such an effect on epithelial cells in the cochlea may cause reduced uptake of loop diuretics in this organ, resulting in reduced toxicity.

Ototoxicity of indacrinone is stereospecific

Indacrinone (MK-196) is a loop diuretic which consists of a racemic mixture. The purpose of this study was to evaluate the individual enantiomers in the chinchilla model to determine whether these compounds affect auditory function and whether a difference in ototoxic potency exists. Very little change of endocochlear potential (EP) or compound action potential (CAP) was noted in animals receiving the (+)-enantiomer. On the other hand, chinchillas injected with the (-)-enantiomer were found to have a dose related reduction in both CAP and EP. These findings suggest the possibility that the diuretic receptor in the kidney and the receptor mediating ototoxicity in the cochlea, may have similar steric requirements for interacting with loop diuretics.

Some organic acids attenuate the effects of furosemide on the endocochlear potential

A series of organic acid transport inhibitors significantly reduced the endocochlear potential (EP) decline produced by furosemide in the chinchilla. Probenecid, sodium salicylate and penicillin G were much more effective than novobiocin, meclofenamate or diatrizoate. Inhibitors of organic base transport, choline and N-methyl nicotinamide, had no effect on the furosemide-induced drop of the EP. These findings suggest that at least part of furosemide ototoxicity may be mediated by organic acid transport.