Characteristics and Potential Biomarkers of Adult Sickle Cell Patients with Chronic Pain
Authors: Albo Camila , Kumar Sanjiv , Pope Michael , Kidwell Kyle Michael , Patel Niren , Wells Leigh , Bowman Latanya , Barrett Nadine , Hongyan Xu , Kutlar Abdullah 
Published: October 2019

Abstract
A paradigm shift is occurring in our understanding of pain in patients with sickle cell disease (SCD). Vaso-occlusive episodes (VOEs) are crises of acute nociceptive pain, and have long been recognized as a hallmark of SCD (Brandow et al., 2017). While patients with SCD are traditionally considered to be at "steady state" and pain free between VOEs, recent studies have shown that a significant number of adults with SCD (~30%) experience daily chronic pain (>50% of the time in the past 6 months) (Smith et al., 2008). Although the precise mechanisms underlying this evolution from acute episodic to chronic pain are not well known, some contributing factors include age, chronic inflammation, organ damage, and opioid induced hyperalgesia (Stoicea et al., 2015; Rees et al., 2010). A recent study in a mouse model of SCD showed that mast cell activation was an important contributor to neurogenic inflammation and chronic pain (Vincent et al., 2013). We previously reported that SCD patients with chronic pain were older and had higher levels of mast cell activation markers, plasma tryptase and substance P, compared to those without chronic pain (Kuei et al., 2015). Recently, nerve growth factor (NGF) has been implicated in pathogenesis of some chronic pain syndromes (osteoarthritis), and clinical trials with anti-NGF monoclonal antibodies have been shown to result in superior pain control compared to placebo, opioids and NSAIDs. Here we report the results of our extended study of the evolution of chronic pain in SCD. A total of 72 subjects (age 15-66) were enrolled: 10 in the 15-19 age group, 19 in 20-29, 21 in 30-39, and 22 in ≥ 40. Patients transfused within the past 3 months and those who had an ED visit or hospitalization within the past 2 weeks were excluded. Information on the frequency of VOEs, presence or absence of chronic pain, HU therapy, opioid use (as mg morphine equivalents within the past 6 months), other medications and routine laboratory data (CBC, retics, chemistry panel, HbF) were collected. 4 mL of EDTA blood was collected at steady state and the plasma was separated by centrifugation and kept at -80 C. Plasma tryptase, substance P, and NGF levels were assayed by ELISA using kits from Biomatik inc (catalog# EKU07922), Enzo Life Sciences (catalog# ADI-900-018), and R&D Systems (catalog# DY256), respectively. Pressure pain threshold (PPTh) was measured using a hand-held digital algometer (AlgoMed, Medoc, Israel) four times at each of the following anatomic muscle groups on the left side of the body and then averaged for analysis: masseter, trapezius, and ulna in this consecutive order. Cutaneous mechanical pain was assessed using a Von Frey monofilament on the back of the subject's left hand. A baseline of one stimulus and then two separate series of 10 repeated stimuli each were conducted. The subject was asked to rate the pain on a scale of 0 to 10 (MFB, MF1 and MF2). Overall, 34 patients had chronic pain and 38 did not; there was an age dependent increase in the frequency of chronic pain, VOE frequency, opioid use and Von Frey MF values. Similarly, QST showed significantly lower pressure pain thresholds in subjects with chronic pain at ulna and trapezius (p=0.026 and 0.024 respectively). As expected, opioid use (daily morphine equivalents) was significantly higher in the chronic pain patients (52.8 mg vs 6.94 mg, p=0.009), suggesting a correlation between opioid use and hyperalgesia. Tryptase and substance P levels were higher in chronic pain patients, though the difference did not reach statistical significance. NGF levels were significantly higher in the chronic pain group (1126 pg/ml vs 473 pg/ml, p=0.051). Our results confirm previous observations that there is an age dependent increase in the proportion of patients with chronic pain (Table 1, Fig. 1-3). The trend towards higher levels of tryptase and substance P is in support of mast cell activation and neurogenic inflammation as a contributing factor to chronic pain (Vincent et al., 2013). To our knowledge, this is the first study of NGF as a possible contributing factor to chronic pain in SCD. If confirmed in larger multi-center studies, these observations could provide a rationale for novel interventions for chronic pain in SCD, via inhibition of mast cell activation/c-kit (tyrosine kinase inhibitors) or via repurposing of existing anti-NGF monoclonal antibodies as an alternative to opioids, whose inefficacy in chronic pain is well documented.

Disclosures
Kutlar: Novartis: Consultancy, Honoraria, Other: Personal fees, Research Funding; Bluebird Bio: Other: DSMB Member; Sancilio: Other: DSMB Chair.