Cisplatin metabolites and their toxicity on isolated cochlear outer hair cells in vitro

Cisplatin otoprotection using transtympanic L-N-acetylcysteine: a pilot randomized study in head and neck cancer patients

OBJECTIVES/HYPOTHESIS

To evaluate the feasibility and efficacy of transtympanic L-N-Acetylcysteine (L-NAC) administration in patients receiving cisplatin chemotherapy for head and neck cancer.

STUDY DESIGN

Prospective randomized nonblinded open-label clinical trial.

METHODS

Transtympanic 2% L-NAC was administered to one randomly selected ear with the other ear as control in each patient. Primary outcome parameter was the difference in the loss of pure tone averages (PTA) at 2, 4, and 8 kHz between the L-NAC and control ear at 1 to 2 months following chemotherapy.

RESULTS

Eleven patients completed the study, with two patients demonstrating significantly better hearing in the L-NAC treated ear (18.2%). However, for the overall group, the difference in hearing preservation did not reach significance. Two percent L-NAC administration was well tolerated in this patient population. There were no adverse effects associated with L-NAC.

CONCLUSION

Although the study did not demonstrate a significant benefit overall, transtympanic L-NAC was associated with significantly better hearing in two patients. Better delivery methods may improve the efficacy of this treatment. L-NAC remains a promising drug in preventing cisplatin-induced ototoxicity.

Protection against cisplatin-induced ototoxicity by N-acetylcysteine in a rat model

Cisplatin (CDDP) is a widely used chemotherapeutic agent that is highly ototoxic. Animal studies and clinical trials have shown that thiosulfates can protect against platinum-induced ototoxicity. This study investigated a new model for CDDP ototoxicity in the rat, and tested the potential chemoprotective effect of administering N-acetylcysteine (NAC) before giving CDDP. Long Evans rats were treated with CDDP 6 mg/kg delivered to the aorta via a retrograde right external carotid artery infusion, 15 min after intravenous (IV) infusion of saline (n=8) or NAC 400 mg/kg (n=8), such that the vertebral arteries were perfused. Subsequent groups were similarly treated with NAC 30 min before (n=7) and 4 h after (n=7) CDDP. Auditory brainstem response (ABR) thresholds were tested at 4-20 kHz, 7 days after treatment and compared to baseline ABR values. The NAC-treated rats exhibited no significant change from baseline values at all time intervals, while the saline-treated rats showed marked ototoxicity, especially at higher frequencies. Furthermore, the rats treated with NAC 15 min before CDDP exhibited less overall toxicity to CDDP, as evidenced in weight loss 7 days post-treatment (mean for saline=-39.63 g; mean for NAC=-21.13 g; p=0.0084). These data show that treatment with NAC can prevent CDDP-induced ototoxicity in rats.

Strategies for prevention of toxicity caused by platinum-based chemotherapy: review and summary of the annual meeting of the Blood-Brain Barrier Disruption Program, Gleneden Beach, Oregon, March 10, 2001

OBJECTIVES

To summarize the findings relevant to otolaryngology from the annual meeting of the Blood-Brain Barrier Disruption Consortium in Gleneden Beach, Oregon, March 10, 2001.

STUDY DESIGN

Summaries are provided by the speakers, as well as related data from the published literature. Findings in otology and oncology regarding ototoxicity that were discussed at the meeting are included.

RESULTS

Data considered included physiological research, animal studies, and clinical trials that relate to platinum-based chemotherapy and prevention of toxicity.

CONCLUSIONS

The dose-limiting side effects of platinum-based chemotherapy are preventable, but questions about the effect of the protective agents on oncological efficacy remain. Strategies for prevention of chemotherapy-induced toxicity include temporal or anatomical separation of cisplatin or carboplatin from sodium thiosulfate, D-methionine, or N-acetyl-cysteine. Clinical application of these methods has begun. The mechanisms presumably involve free radicals or drug conjugation, or both. Understanding the role of free radicals in medicine is likely to become important in the future.

Round window application of D-methionine provides complete cisplatin otoprotection

OBJECTIVE

Cisplatin is a widely used, very effective chemotherapeutic agent that can cause severe ototoxicity. In this study, D-methionine was tested as an otoprotectant via round window membrane (RWM) application in the chinchilla.

METHODS

A minute amount of cisplatin alone, or D-methionine followed by cisplatin, was applied topically directly to the intact RWM of anesthetized adult chinchillas. Auditory brainstem responses were measured before and 1 week after topical round window application. Animals were killed, and the cochleas were examined.

RESULTS

The ears pretreated with D-methionine were completely protected from hearing loss and hair cell loss in the organ of Corti compared with controls. The ears receiving cisplatin without D-methionine protection sustained nearly complete hearing loss with threshold shifts of >60 dB, with extensive outer hair cell loss throughout the organ of Corti but particularly in the basal turn.

CONCLUSION

These results demonstrate that topical D-methionine provides excellent otoprotection against cisplatin-induced ototoxicity both electrophysiologically and structurally.

Disruption of mdr1a p-glycoprotein gene results in dysfunction of blood-inner ear barrier in mice

P-glycoprotein (p-gp), a drug transporter in multidrug-resistant cancer cells, is a transmembrane protein encoded by mdr1a, mdr1b and mdr2 genes in mice. In our previous report, high level p-gp was immunohistochemically detected in capillary endothelial cells of the guinea pig inner ear, supporting a possible role as an extrusion pump in the blood-inner ear barrier (BIB). We investigated the functional involvement of p-gp in the inner ear using mdr1a gene knock-out mice [mdr1a(-/-) mice]. Pharmacokinetic analyses showed that mdr1a(-/-) mice displayed obviously increased accumulations of the p-gp-transported drugs doxorubicin (adriamycin, ADM) and vinblastine in the inner ear tissues compared with those in mdr1a(+/+) mice. Subsequent functional studies using auditory-evoked brainstem responses showed hearing impairment only in mdr1a(-/-) mice after administering these drugs. Furthermore, inhibition of p-gp function by co-administration of cyclosporin A (CsA) with doxorubicin (ADM) in mdr1a(+/+) mice resulted in increased accumulation of ADM in inner ear tissues and hearing impairment similar to that noted in mdr1a(-/-) mice. We conclude that mdr1a p-gp, which acts as an efflux pump in the inner ear, prevents ototoxicity induced by p-gp substrate drugs and contributes to a new functional mechanism in the BIB.

A radical demise. Toxins and trauma share common pathways in hair cell death

The pathologic similarities noted after ototoxic and/or traumatic injury to the cochlea as well as the key features of the cochlea that make it susceptible to reactive oxygen species (ROS) damage are reviewed. Recent evidence linking ROS to cochlear damage associated with both ototoxins and/or trauma are presented. Mechanisms of generation of ROS in the cochlea and how these metabolites damage the cochlea and impair function are also reviewed. Finally, examples of novel therapeutic strategies to prevent and reverse hearing loss due to noise and/or ototoxins are presented to illustrate the clinical relevance of these new findings.

The electroreceptor organ of the catfish, Ictalurus melas, as a model for cisplatin-induced ototoxicity

The ototoxic side-effects of the anti-cancer drug cisplatin (cis-diaminedichloroplatinum) have been widely investigated. However, the exact site of action remains unclear. In this study, the electroreceptor organ of the freshwater catfish Ictalurus melas is used as a model for examining the acute effects of cisplatin. The sensory cells in the electroreceptor organ are homologous to the inner hair cells in the cochlea of mammals. The effects of cisplatin administration can be investigated by in vivo recording of the spike trains from the electroreceptor organ primary afferents. Exposure of electroreceptor organs to 330 microM cisplatin for 1 h causes the spontaneous activity to drop, the overall sensitivity to diminish and the shape of the frequency characteristics to change. These effects persist in the week after administration. Control levels have returned at day 22. These results demonstrate an acute and, with considerable hysteresis, reversible cisplatin effect on the electroreceptor organs, which is to a large extent consistent with the cisplatin-induced effects in isolated hair cells in mammals. The time-course of the effect supports the hypotheses that ion channels are blocked immediately by cisplatin administration, and that cisplatin metabolites disturb enzymatic cellular processes.

Effect of 4-methylthiobenzoic acid on cisplatin-induced ototoxicity in the rat

The purpose of this study was to investigate the effectiveness of 4-methylthiobenzoic acid (MTBA) as a protection agent against cisplatin (CDDP)-induced changes in organ of Corti surface structure, compared to electrophysiological changes. Electrophysiological change was assessed using auditory brainstem response (ABR) and morphological changes were assessed using scanning electron microscopy (SEM). Male Wistar rats underwent pre-treatment ABRs in response to clicks, and tone bursts at 2, 4, 8, 16, and 32 kHz. The three groups of rats were injected as follows: (1) MTBA (250 mg/kg, i.p.), (2) CDDP (16 mg/kg, i.p.), (3) CDDP+MTBA (16 mg/kg, i.p. + 250 mg/kg, i.p.). Post-treatment ABRs were performed 3 days after drug administration and rats were sacrificed. Their cochleae were harvested and SEM was used to examine the surface of the organ of Corti, specifically the number of inner hair cells (IHCs) and outer hair cells (OHCs) in the apical, middle and basal turns of the cochlea. Animal weight was measured on the first and final days. There was a good correlation between ABR threshold changes and hair cell loss in the high frequency region of the cochlea (basal turn), while threshold changes in the lower test frequencies (middle turn) appeared to be the result of more subtle changes in the cochlea. MTBA provided effective protection against cisplatin-induced ABR threshold changes at all test frequencies as well as hair cell loss. MTBA also protected against body weight loss.

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.

Endothelial injury of capillaries in the stria vascularis of guinea pigs treated with cisplatin and gentamicin

The drugs cisplatin and gentamicin are used for treatment of various cancer patients suffering from infection. The authors report a detailed electron microscopic study of blood vessels in stria vascularis of guinea pigs after treatment with cisplatin alone and in combination with gentamicin. The most distinctive features expressing endothelial cellular injury were mitochondrial, including occasional paracrystalline inclusions; electron-lucent foci with depleted organelles; intracytoplasmic vacuole formations; lipid bodies; cytoplasmic extrusions located on the luminal surface; and severe luminal constriction of part of the vessels from animals treated with the combined drugs. The study suggests that the damage to strial capillaries due to treatment with cisplatin alone and in combination with gentamicin may contribute to the injurious effects of these drugs on the strial tissue. Furthermore, the results of this study may enlarge the awareness of the potential vascular damage and vascular complications in additional body systems after medical use of cisplatin alone or in combination with gentamicin.

Similar pharmacokinetics and differential ototoxicity after administration with cisplatin and transplatin in guinea pigs

Transplatin is a transisomer of cisplatin. Although cisplatin exhibits strong ototoxicity, there is no report concerning the ototoxicity of transplatin. To evaluate differences in pharmacokinetics and ototoxicity, cisplatin (7.5 mg/kg) and transplatin (30 mg/kg) were administered to guinea pigs twice at an interval of 5 days. The N1 threshold of the compound action potential was significantly elevated after administration of cisplatin. Cochleogram of the cisplatin-treated group showed severe losses of outer hair cells essentially at the basal and second turns. Transplatin, however, did not induce any detectable functional or morphological changes. Furthermore, blood urea nitrogen and creatinine levels in serum were not elevated after administration of transplatin, whereas cisplatin showed strong nephrotoxicity. The serum and perilymphatic concentrations of platinum up to 24 h after administering an equimolar dose of cis- or transplatin (7.5 mg/kg) were almost similar. As has been reported by many investigators, transplatin has no anti-tumor activity because stereochemical limitations preclude transplatin from forming intra- and interstrand closs-links with nuclear or mitochondrial DNA. From these results, it was concluded that the stereochemical structure of the platinum compound and steric interaction with target molecules such as mitochondrial DNA in the cochlear outer hair cells might be important to the ototoxic mechanism of platinum compounds.