Characteristics and Potential Biomarkers for Chronic Pain in Patients with Sickle Cell Disease

Vaso-occlusive episodes (VOE) or pain crises are a hallmark of sickle cell disease (SCD), with increasing recognition that a significant portion of SCD patients develop chronic pain. In the landmark PiSCES study (Smith et al), patients reported pain on 55% days, with ~30% reporting pain on >90% days. Thus, the episodic, nociceptive pain (VOE) in younger patients, evolves into a chronic pain syndrome, with neuropathic and centralized components in some adults. Kutlar et al (Blood, 2014), reported on the association of different pain related phenotypes (pain diaries, frequency of hospitalizations/ED visits, pressure pain threshold) with polymorphisms in candidate genes in 167 SCD patients, providing evidence that multiple signaling pathways and mechanisms are likely involved.

In this study, 12 SCD subjects with "chronic pain", defined by reported pain >50% of days in pain diaries collected over 6 months, were enrolled (SCD-CP). 17 SCD patients who did not have chronic pain (SCD-NCP), and 9 non-SCD African-Americans (C) were enrolled as controls. Informed consent was obtained. Age, gender, Hb F levels, HU usage, and pressure pain algometer readings were recorded from SCD subjects. 8 ml of blood (EDTA) was collected from subjects at "steady state" and from normal controls. Plasma was separated and kept at -80 C until the assay. Plasma tryptase and Substance P levels were assayed by ELISA using kits from Biomatik, Inc. (catalog # EKU07922) and Enzo Life Sciences (Catalog #ADI-900-018), respectively. SCD-CP patients were significantly older than SCD-NCP: mean age 41 vs 32.2 (p=0.033). The pressure pain algometer readings did not differ significantly between SCD-CP and SCD-NCP at three sites (trapezius, ulna, masseter, p= 0.67-0.74). There were 12/17 patients on HU (70.6%) among SCD-NCP, and 6/12 (50%) among SCD-CP (p=0.435). Similarly, Hb F levels were not significantly different (14.7% in SCD-CP, vs 11.7% in SCD-NCP, p=0.446). Opioid use (average morphine equivalent as mg/day) was significantly higher in the chronic pain group (11.45 mg/day, vs 2.92 mg/day, p=0.015). Plasma tryptase and substance-P levels are shown in the table:

The difference in substance-P levels did not reach significance across groups by ANOVA (p=0.337) or in pairwise comparison between groups. However, tryptase levels were significantly different across groups by ANOVA (p=0.00615). Pairwise comparisons between two groups showed that tryptase levels were significantly different between SCD-CP and controls (p=0.0053).

These results highlight characteristics of SCD patients with chronic pain: they are older, have a higher use of opioids, and have significantly higher tryptase levels. These observations support previous findings that age is a significant factor in transition to chronic pain in SCD. Higher dose of opioid use in SCD-CP could result from dose escalation to control pain; conversely, it could be argued that higher opioid use itself could be a factor in development of chronic pain through opioid-induced hyperalgesia. To our knowledge, this is the first study of plasma tryptase levels in SCD, in relation to different pain phenotypes. Tryptase is released into plasma with degranulation of mast cells and leads to inflammation, anaphylaxis, urticaria, and neuropathic pain. It binds PAR2 (protease activated receptor 2), releasing inflammatory mediators and substance P, inducing neurogenic inflammation. Elevated tryptase levels are found in systemic mastocytosis, and the newly recognized Mast Cell Activation Syndrome (MCAS). Vincent et al (Blood, 2013) showed that mast cell activation played an important role in neurogenic inflammation and chronic pain in a mouse model of SCD. They also demonstrated that inhibition of mast cell activation, via c-kit knockout or with imatinib or cromolyn sodium improved neurogenic inflammation and chronic pain. Two recent case reports (Murphy et al, Stankovic et al) document significant improvement in pain in SCD patients who developed CML, during treatment with imatinib. These observations, and the findings of our pilot study, not only suggest a novel mechanism and biomarker for chronic pain in SCD, but also a potential therapeutic target by inhibition of mast cell activation via c-kit pathway, or stabilization with cromolyn.