Author:

Abstract:

For many cancer survivors, the burden of the disease extends beyond successful treatment. Often, chronic side effects affect patient's quality of life after treatment completion, e.g. chronic fatigue, fertility problems or chemotherapy-induced peripheral neuropathy (CIPN). The latter concerns a debilitating pathology in patients treated with specific chemotherapeutics agents, including platinum derivatives (oxaliplatin, cisplatin), taxanes (paclitaxel, docetaxel), vinca alkaloids (vincristine), or proteasome inhibitors (bortezomib). Symptoms typically include numbness, paraesthesia, and thermal or mechanical hypersensitivity. Unfortunately, effective preventive, curative or symptomatic treatment is lacking. In search thereof, rodent models elicited a pivotal role for transient receptor potential (TRP) channels, TRP Ankyrin 1 (TRPA1) and TRP Vanilloid 1 (TRPV1) in particular. However, whether these preclinical observations translate into functional in vivo alterations in patients with CIPN, remained to be investigated. Therefore, the general objective of this research project was to evaluate the in vivo TRP functionality in rodent models of CIPN as well as in patients suffering from CIPN after treatment with either oxaliplatin or paclitaxel. The overarching methodology included the quantification of the vascular response upon TRPA1 and TRPV1 activation on peripheral nerve endings. TRPA1 and TRPV1 were activated by topical application of cinnamaldehyde or allyl isothiocyanate (AITC) and capsaicin, respectively. The dermal blood flow (DBF) increase was quantified using laser imaging techniques. Prior to its implementation in rodent models of CIPN or in a patient setting, the methodology was optimized. In particular, the vascular response upon topical capsaicin, cinnamaldehyde and AITC application was characterized on mice (CD1 or C57BL/6J) ears, including the reproducibility and TRP specificity. Although capsaicin induced a robust, reasonably reproducible (ICC = 0.557) and TRPV1 specific DBF increase, the dose-dependent response upon cinnamaldehyde was poorly reproducible (ICC = 0.362) and no TRPA1 specificity could be withhold. Similarly, the dose-dependent response upon AITC in ethanol was poorly reproducible within a single animal (ICC 0.225) and could only be abolished in TRPA1-/-TRPV1-/- double knock-out mice. Thus, for further preclinical experiments in which the in vivo TRP functionality was assessed, Sprague Dawley rats were used instead of mice. In addition, since all TRP functionality assessements in humans have previously been performed on the forearm whereas the symptoms of patients with CIPN primarily manifest at the hands or feet, the DBF response upon topical cinnamaldehyde and capsaicin application was characterized on the proximal phalanges of the fingers. Topical application of either challenge agent induced a dose-dependent DBF increase on fingers. Besides, the inter-hand and inter-period reproducibility was similar on fingers and forearms. This technique was thus considered to be a safe and feasible method to investigate the in vivo TRPA1 and TRPV1 functionality on distal peripheral nerve endings in patients with CIPN. To evaluate the in vivo TRP functionality in patients with CIPN, age-matched healthy control groups were composed in order to correct for a declince in (peptidergic) nerve fiber functionality. In particular, the DBF increase upon capsaicin application was attenuated in healthy men aged 70-75 years compared to men aged 20-25 years (42%). Likewise, also the flare response upon cinnamaldehyde application was reduced (57%). Next, the in vivo TRP functionality in CIPN was evaluated. First, in addition to an increased response to cinnamaldehyde in cultured dorsal root ganglia (DRG) neurons, also the cinnamaldehyde-induced DBF response was enhanced in oxaliplatin- versus vehicle-treated rats (138%). Moreover, the enhanced DBF response was also observed in patients with oxaliplatin-induced peripheral neuropathy (OIPN) compared to healthy controls (263%), providing translational proof that the increased TRPA1 functionality persists in vivo, and underpinning the hypothesis that patients with OIPN could truly benefit from TRPA1 antagonists. In contrast, patients who suffered from chronic paclitaxel-induced peripheral neuropathy (PIPN) showed no robust DBF increase from baseline upon cinnamaldehyde application. Besides, the DBF increase upon capsaicin application was reduced compared to healthy controls (44%). Moreover, during a 12-week treatment period of weekly paclitaxel administrations to combat breast cancer, the vascular response to cinnamaldehyde diminished even before any clinical symptoms were present. Likewise, also the capsaicin response was reduced after treatment completion. Thus, rather than an increased TRP functionality, PIPN is clinically characterized by a loss of intraepidermal nerve fibers (IENF). To ensure that the reduced response was not due to a general anti-inflammatory effect of concomitant glucocorticoid intake, the effect of oral intake of dexamethasone 10 mg to the vascular response upon either cinnamaldehyde or capsaicin application was assessed. However, dexamethasone did not affect either response. Finally, to investigate whether specific marketed drugs show any TRPA1 and/or TRPV1 antagonism, as was suggested in preclinical research, the effect of paracetamol, metamizole and etodolac was evaluated. However, none of these drugs interfered with the in vivo TRP functionality in human. In contrast, the observed inhibition (- 30%) of the DBF increase upon cinnamaldehyde application after a multiple dose (4 x 500 mg over 24 hours) of metamizole highly reflected the inhibition after more classical nonselective cyclo-oxygenase (COX-) inhibitors such as ibuprofen and indomethacin. In conclusion, the current research project confirmed that TRPA1 functionality is not only increased in isolated DRG neurons in rodents, yet also in vivo in rats and in patients with OIPN. Thereby, the results provide an important incentive to conduct a phase-II clinical trial with TRPA1 antagonists in this patient population. In contrast, PIPN is rather characterized by a loss of IENF. Whether or not the IENF loss goes hand in hand with an increased TRPV1 functionality could not be confirmed. Yet, instead of TRP antagonists, patients with PIPN would most probably rather benefit from a treatment that preserves or restores the peripheral nerve integrity.