Substance P and Sickle Cell Disease—A Marker for Pain and Novel Therapeutic Approaches

The mechanisms of pain in sickle cell disease (SCD) and its acute, recurrent, chronic, and neuropathic pain syndromes have been an enigma since their initial clinical descriptions more than a century ago. The report in this issue of the Journal by Brandow et al (2016) confirms our earlier observation of elevated plasma substance P (SP) in patients with SCD (Michaels et al 1998). Substance P is an undecapeptide tachykinin which initiates biological effects following its interaction with its preferred receptor, Neurokinin 1, which is a G-protein coupled receptor. These findings not only suggest that plasma SP levels may be an important measure for pain in SCD, but also that a clinical trial of a neurokinin-1 receptor antagonist would be a potential therapeutic approach toward treating SCD pain. The initial observations among Africans, and the observation of SCD children who have resistance to malaria and leg ulcers, were among the earliest clinicopathological discoveries related to SCD. There have been exciting and unique developments in the studies of the biology of pain in SCD and its mechanisms, and these investigators offer new insights into its pathophysiology (Ballas et al 2012; Brandow et al 2015). Brandow et al (2015) undertook a systematic review of the neurobiology of pain in SCD. In murine SCD models, inhibition in three pathways involving different mechanisms reversed mechanical, cold and heat hypersensitivity. Inhibition was demonstrated by cannabinoid receptor antagonists the mast cell inhibitors cromolyn sodium and imatinib, and the mechanical pain receptors, the TRPV family of transient receptor potential cation channels. A single clinical trial of trifluoperazine, an anti-psychotic that inhibits calcium calmodulin protein kinase II alpha, was done in 18 SCD adults and this drug showed safety and decreased pain intensity (Reviewed in Brandow et al 2015).

Elevated plasma levels of SP have been implicated as a possible component in the pathophysiology of SCD (Douglas 2008; Michaels et al 1998). A group of patients had elevated SP levels at baseline, compared to controls and plasma SP levels were further increased during acute SCD crisis. This study included 30 well children, aged 2 to 18 years old with SCD, as well as 21 children with SCD in pain crisis and age-matched controls (Douglas 2008). SP as well as interleukin 8 levels were increased in SCD, as compared with the control group. The plasma tumour necrosis factor (TNF) levels were similar in the groups of patients in this study. It was speculated that SP is important in pain and inflammation in SCD and may be a measurable laboratory marker of vaso-occlusive crises (Douglas 2008; Michaels et al 1998). Further, it was proposed that treatment with a neurokinin-1 receptor antagonist may have a therapeutic role in the treatment of SCD crisis pain (Douglas 2008; Michaels et al 1998). The recent paper by Brandow et al. (2016) has confirmed and extended these findings. In their study, measured plasma SP levels were elevated in SCD, and these elevations correlated with haemolysis. In addition, patients taking hydroxycarbamide had increased plasma SP levels. In their provocative paper, Brandow et al. (2016) raise further, important questions about the role of SP, not only as a biomarker, but also of pathophysiological significance in nociceptor sensitization leading to increased release of SP. The complex associations between plasma SP, neuropathic pain, haemolysis, and downstream pathophysiology are elegantly considered (Ballas et al 2012). In addition, Brandow et al (2016) report the association of haemolysis with nitric oxide depletion and oxidative stress, which can in turn potentially impact peripheral and/or central nociceptor sensitization and chronic neuropathic pain in a vicious cycle. Moreover, it has recently been demonstrated that SP treatment of monocyte-macrophages ex vivo favours upregulation of the CD163 surface receptor, the haemoglobin-haptoglobin receptor and leads to a proinflammatory macrophage polarization phenotype (Tuluc et al 2014).

Striking, and of significance, recent studies have demonstrated in a mouse model that mast cell activation contributes to SCD pathophysiology and pain, associated with this activation (Vincent et al 2013). These SCD mice have elevated plasma SP levels. Further treatment of these animals with a high-affinity small molecule (AT200), a nociceptin opioid receptor agonist, ameliorates mast cell activation and pain in this murine SCD model (Vang et al 2015).

Moreover, and of direct relevance, the studies of SP and Neuorkinin-1 (Barret et al 2016, Douglas 2008; Douglas & Leeman 2011; Douglas et al 2016; Michaels et al 1998;Tebas et al 2001; Tebas et al 2015), and the findings of Brandow et al (2016), are the observations of elevated plasma SP in the HBSS-BERK mouse, designated as the transgenic sickle cell mouse (Vincent et al 2013). HBSS-BERK is the sickle cell mouse with murine α and β globin knockouts that expresses human sickle or normal haemoglobin A. Taken together, these investigations support the proposal that a unique approach to the treatment of chronic pain and pain sensitization in SCD may be to utilize SP as a potential biomarker for SCD pain and vaso-occlusive crises, as suggested by both Douglas (1998) and Brandow et al (2016). The preferred tachykinin receptor for SP is neurokinin-1. Reductions in plasma SP levels and pro-inflammatory cytokines, including granulocyte colony-stimulating factor (G-CSF), interleukin (IL)-6, IL8 and TNF-α, have been demonstrated in Phase IB clinical trials of a neurokinin-1 receptor antagonist in human immunodeficiency virus (HIV) (Tebas et al 2001; Valverde et al 2016) (NIH clinicaltrials.gov; NCT00428519; NCT01300988). The clinical efficacy of neurokinin-1 receptor antagonists are contingent on receptor occupancy, protein binding and complex pharmacometric analysis, which we have recently investigated in relationship to the potential use of aprepitant, as adjunctive therapy in HIV-infected subjects (Barret et al 2016). Aprepitant (Emend) is the only US Food and Drug Administration (FDA)-licensed neurokinin-1 receptor antagonist and has been licensed for chemotherapy-induced nausea and vomiting (CINV). Fos-aprepitant has licensure as an intravenous agent used for acute post-operative emesis and pain. Further developments of novel neurokinin antagonists, as well as other tachykinin inhibitors, are being investigated (Douglas et al 2016).

The pathogenesis of sickle cell pain and crises, despite many years of clinical research, remains complex. The role of nitric oxide depletion, oxidative stress and nociceptor sensitization are all important components. SP and reactive oxygen species are elevated in the spinal cords of sickle cell mice and curcumin, or coenzyme Q 10, decreases SP levels and reduces pain sensitivity (Tebas et al 2015). In conclusion, future approaches toward SCD pain mechanisms for therapy should consider the use of SP as a biomarker and, further, the potential use of neurokinin-1 antagonists as a therapeutic approach to pain and co-morbidities in sickle cell disease.