Acute Abnormalities of Sensory Nerve Function Associated With Oxaliplatin-Induced Neurotoxicity Susanna B. Park, David Goldstein, Cindy S.-Y. Lin, Arun V. Krishnan, Michael L. Friedlander,and Matthew C. Kiernan JOURNAL OF CLINICAL ONCOLOGY VOLUME 27 NUMBER 8 MARCH

INTRODUCTION Oxaliplatin a third-generation platinum analog first-line chemotherapy in metastatic colorectal ca. & adjuvant therapy unique spectrum of neurologic symptoms Acute neurotoxicity develops immediately after infusion cold-exacerbated paresthesia, muscle spasms, fasciculations acute symptoms typically resolve within a week at higher cumulative doses, oxaliplatin induces dose-limiting sensory neuropathy leading to sensory ataxia and functional impairment Severe oxaliplatin-induced neuropathy occurs in 10% to 20% of patients receiving greater than 750 to 850 mg/m 2

Neurotoxicity limits treatment tolerability treatment delay or cessation neuropathic symptoms may persist long term Attempts to prevent oxaliplatin-induced neurotoxicity :Not successful The pathophysiologic mechanisms > unclear Recent in vitro studies Oxaliplatin acts directly on axonal membrane voltage-gated sodium (Na) channels The exact mechanisms underlying acute oxaliplatin-induced neurotoxicity >> unclear & undefine A better understanding of the mechanisms of oxaliplatin neurotoxicity could assist in the development of effective neuroprotective strategies

Clinical grading scales the National Cancer Institute Common Toxicity Criteria for Adverse Events (NCI-CTCAE) commonly used to monitor and rate severity of chemotherapy induced neurotoxicity Conventional neurophysiologic techniques nerve conduction studies (NCS) do not clear provide insight into pathophysiologic mechanisms underlying neurotoxicity limited utility in monitoring the development of toxicity

The present study used potentially more sensitive axonal excitability techniques, recently adapted for clinical use, to investigate oxaliplatin- induced neurotoxicity. Patients were observed prospectively across treatment and assessed before and after individual oxaliplatin infusions to better define mechanisms involved in the development of oxaliplatin-induced neurotoxicity.

PATIENTS AND METHODS Study Design Patients with colorectal cancer (stage II, III, or IV) routinely referred from the Department of Medical Oncology, Prince of Wales Hospital (Sydney, Australia) for clinical neuropathy grading, conventional NCS, and assessment of axonal excitability. Excluded a history of peripheral neuropathy received prior neurotoxic chemotherapy baseline NCS revealed peripheral neuropathy

Patients received one of the following treatment regimens standard infusional fluorouracil, leucovorin, and oxaliplatin (FOLFOX 4; oxaliplatin dose of 85 mg/m 2 ) infusional fluorouracil, leucovorin, and oxaliplatin (FOLFOX 6; oxaliplatin dose of 100 mg/m 2 ) capecitabine plus oxaliplatin (XELOX; oxaliplatin dose of 130 mg/m 2 ) Treatment was continued until maximal clinical benefit was reached or unacceptable toxicity developed. Treatment delays and dose reductions were managed according to standard clinical practice.

Assessment of Neurotoxicity Neurotoxicity was graded according to the NCI-CTCAE version 3 using the neuropathy sensory subscale with the following grading system: grade 1 : loss of deep tendon reflexes or paresthesia not interfering with function grade 2 : sensory alteration or paresthesia interfering with function but not activities of daily living grade 3 : sensory alteration or paresthesia interfering with activities of daily living grade 4 : disabling

The Oxaliplatin-Specific Neurotoxicity Scale was used as a specific measure of oxaliplatin-induced neurotoxicity with the following grades: grade 1, dysesthesia or paresthesia that completely regresses before the next cycle of therapy grade 2, dysesthesia or paresthesia persisting between courses of therapy grade 3, dysesthesia or paresthesia causing functional impairment Standard NCS incorporating measures of the tibial, sural, median, and radial nerves were performed using a Medelec Synergy system (Oxford Instruments, Abingdon, United Kingdom).

Sensory axonal excitability testing : according to previously detailed protocols using QTRAC software (Institute of Neurology, London, United Kingdom). The median nerve was stimulated at the wrist, and compound sensory action potentials (CSAPs) were recorded from the second digit. Peak CSAP amplitude was recorded during each treatment cycle and obtained before each excitability recording. Multiple excitability parameters were recorded including refractoriness and superexcitability, which are parameters that assess the recovery of excitability after impulse generation and indicate the function of voltage- gated ion channels. Refractoriness and superexcitability were measured as the percent change in threshold after a supramaximal conditioning stimulus at interstimulus intervals of 2.5 and 7 msec, respectively

RESULTS Clinical Characteristics Between August 2005 and September 2007, 25 patients with colorectal cancer were enrolled onto the study Clinical characteristics Excitability recordings were undertaken before oxaliplatin infusion and within 2 to 3 days after infusion across 88 cycles of treatment to assess immediate changes in axonal excitability.

The majority of patients developed acute sensorimotor neuropathic symptoms (92%), including prominent cold-associated paresthesia (88%), jaw tightness or pain (20%), and muscle cramp or fasciculations (12%). Incidence of neurotoxicity was recorded using NCI-CTCAE and Oxaliplatin- Specific Neurotoxicity Scale (Fig 1B). By completion of treatment, 16% of patients had developed grade 3 sensory neurotoxicity with paresthesia and numbness interfering with function, consistent with previous reports. No patient reported chronic motor symptoms.

Oxaliplatin dose reduction or cessation as a result of neurotoxicity was required in 40% of patients. Conventional NCS were performed after completion of oxaliplatin treatment to confirm the development of clinical symptoms (Table 2). In a subgroup of 16 patients examined 77 days after treatment with a mean cumulative oxaliplatin dose of 810mg/ m 2, 13 patients displayed a generalized sensory neuropathy of the axonal type with generalized reductions in CSAP amplitudes and relative preservation of sensory conduction velocity. Motor studies remained normal.

Acute Oxaliplatin-Induced Abnormalities in Axonal Excitability To investigate the development of oxaliplatin-induced neurotoxicity, sensory axonal excitability recordings were compared before and after infusion in each patient. Postinfusion recordings revealed significant alterations in measures related to Na channel function (Fig 2A). Refractoriness values were significantly decreased after infusion (Fig 2B), suggestive of an alteration in Na channel kinetics. Reduction in refractoriness was associated with a corresponding decrease in superexcitability (Figs 2A and 2B).

Comparison of recordings obtained in early treatment with later recordings indicated greater acute change in Na channel parameters in earlier treatment (Fig 2C) Pre- and postinfusion differences in refractoriness were pronounced in early treatment. Recordings taken before and after infusion in later treatment demonstrated a reduction in these acute changes, suggesting that immediate changes might be masked by the development of chronic nerve dysfunction in later cycles.

In support of this hypothesis a significant reduction in CSAP peak amplitude by the final treatment (Fig 3) associated with an increase in the stimulus intensity required to generate an impulse of a defined size, suggesting the development of sensory axonal loss. Acute changes in axonal excitability parameters that developed in early treatment cycles seemed to anticipate the development of later neurotoxicity in patients who completed seven or more treatment cycles. Patients who completed treatment with moderate or severe neurotoxicity (NCI-CTCAE grade 2 or 3; 11 patients) displayed greater changes in early treatment (cycle 1 or 2), particularly reductions in superexcitability.

Patients who completed treatment with no or mild neurotoxicity (NCI-CTCAE grade 0 or 1; six patients) did not demonstrate significant reductions in superexcitability. On an individual patient basis, a reduction in superexcitability in initial treatment cycles correctly predicted eight (72%) of 11 patients who subsequently developed moderate or severe neurotoxicity, whereas an initial increase in superexcitability correctly predicted five (83%) of six patients with no or mild neurotoxicity (Fig 4B), suggesting that the degree of acute oxaliplatin-induced nerve dysfunction may predict neuropathy severity at treatment completion.

DISCUSSION In this prospective analysis of oxaliplatin-induced neurotoxicity, we examined the role of clinical excitability testing in identifying early changes in nerve function associated with the development of severe presentations of neurotoxicity. We demonstrated oxaliplatin administration acutely modulated peripheral nerve excitability excitability testing may provide quantitative neurophysiologic assessment of acute oxaliplatin-induced sensory nerve dysfunction in a typical clinical setting. Assessment of axonal excitability indicated Na channel alterations after infusion, supporting a role for Na channel dysfunction in the etiology of oxaliplatin-induced neurotoxicity.

Importantly, the degree of early change may relate to the severity of subsequent neurotoxicity, suggesting a link between acute and chronic presentations of oxaliplatin-induced neurotoxicity. Neurotoxicity is the major cause of dose reduction and discontinuation of oxaliplatin treatment, with severe neurotoxicity occurring in 15% to 20% patients with a cumulative dose of 750 to 850 mg/m 2.

Conventional NCS confirmed chronic oxaliplatin-induced neurotoxicity is characterized by the development of axonal sensory neuropathy with reduction in compound sensory amplitudes relative preservation of conduction velocity no changes in motor amplitudes Patterns of excitability change typically reflect underlying modulation in axonal membrane potential and ion channel function. Refractoriness has been classically associated with nodal Na channel function, reflecting changes in inactivation and gating properties.

Qualitatively similar findings in refractoriness tetrodotoxin, a marine toxin with potent Na channel– blocking properties, and another Na channel blocker the local anesthetic mexiletine ; partial blockade of Na channels produces a similar pattern of changes to those demonstrated acutely after oxaliplatin treatment Genetic mutations in SCN1B Na channel subunits produce alterations in both refractoriness and superexcitability reflecting the close association of these parameters as measures of the recovery of excitability after an impulse

In vitro studies (Although specific mechanisms remain unclear) a role for Na channels in oxaliplatin-induced neurotoxicity attributed to slowing of channel inactivation kinetics, alteration in voltage dependence, or oxalate-mediated calcium chelation Alterations in sensory axonal excitability occurred in early treatment (within the first month of a 6-month treatment regimen) and were greater in patients who subsequently developed more severe chronic neurotoxicity. Patients with acute reductions in superexcitability in early treatment were more likely to complete treatment with moderate or severe neurotoxicity, suggesting that acute neurotoxicity may be linked to the development of chronic symptoms.

Na channels central determinants of axonal excitability disruption of normal Na channel activity has widespread consequences on axonal function Acute Na channel dysfunction and aberrant Na ion influx axonal degeneration and loss in multiple models of nerve damage including toxic and inflammatory mechanisms Sustained impairment of Na channel function result in excess calcium (Ca2) influx and axonal injury via reversal of Na/Ca2 exchangers and other processes leading to activation of destructive biochemical cascades and axonal degeneration

Platinum-based compounds dorsal root ganglion (DRG) damage & neuronal cell death Cronic sensory neurotoxicity may reflect damage to DRG neurons, which are relatively poorly protected by the blood-nerve barrier. Although the effects of oxaliplatin on DRG Na channel function cannot be directly assessed in the clinical setting, the results of the present study suggest that peripheral nerve alterations in Na channel properties may represent an accessible marker of neuronal dysfunction.

Several studies of putative neuroprotective agents in oxaliplatin-treated patients have suggested that reduction in acute symptoms and reduction in chronic neuropathy may be linked. In addition, genetic polymorphisms have been identified that confer increased risk of neurotoxicity in oxaliplatin treated patients,suggesting that acute changes in axonal function associated with oxaliplatin treatment may reflect an underlying predisposition to develop chronic neurotoxicity.

Several neuroprotective strategies have been examined in oxaliplatin- induced neurotoxicity with limited success. Strategies to modulate axonal Na channels mixed results different approaches may be required to specifically target oxaliplatin- induced modulation of Na channel function. Improved understanding of pathophysiologic mechanisms underlying neurotoxicity will inevitably assist in the development of appropriate neuroprotective approaches

Conventional NCS remain the gold standard neurophysiologic assessment to measure the extent of peripheral nerve damage compound amplitudes often decline late in oxaliplatin treatment once significant nerve damage has already occurred. The development of a prognostic marker of neurotoxicity severity would enable targeting of at-risk patients for careful monitoring or delivery of future neuroprotective strategies. Additionally, such techniques may enable patient-specific dosing schedules to maximize cumulative dose without incurring potentially irreversible nerve damage.

CONCLUSION Oxaliplatin-based chemotherapy is widely used to treat patients with metastatic colon cancer and is now considered the standard of care in the adjuvant setting. Neurotoxicity is an important and potentially disabling adverse effect of oxaliplatin that can limit the number of cycles of treatment and have important late effects that impact on quality of life and survivorship. Recent studies have highlighted the long-term significance of neurotoxicity, with 10% of patients experiencing neurotoxicity lasting more than 2 years. Reversibility is a critical issue in the adjuvant setting, where treatment benefit must be weighed against risk of unacceptable toxicity. The development of novel techniques to facilitate early identification of oxaliplatin-induced neurotoxicity will be crucial to reduce the burden of sustained neurotoxicity and expedite development of neuroprotective agents.