The Neuropeptide Substance P Is Elevated in Patients with Sickle Cell Disease

Patients with sickle cell disease (SCD) experience increased pain to thermal (cold, heat) stimuli compared to healthy controls suggesting peripheral and/or central sensitization. However, the underlying etiology of this thermal hypersensitivity is unknown. Substance P (SP), a primary pain neurotransmitter, is a potent mediator of neurogenic inflammation where inflammatory cell activation, vasodilation, and edema occur after the stimulation of sensory nerve fibers. Chronic elevation of SP can sensitize peripheral nociceptors resulting in thermal hypersensitivity and chronic neuropathic pain as shown in mice and humans without SCD and in SCD mice. Little is known about SP levels in patients with SCD in baseline health compared to healthy African American controls. Furthermore, little is known about changes in SP levels during acute SCD pain. The objective of this study was to evaluate plasma SP levels in SCD patients and controls in baseline health and in SCD patients during acute pain. We hypothesized that SCD patients, in baseline health, will have significantly higher SP levels compared to controls suggesting chronic pain sensitization and that SP levels will significantly increase during acute pain. 

Patients with SCD in baseline health and during admission for acute pain and healthy African American controls were recruited. Our primary outcome was plasma SP level measured in 3 independent cohorts: healthy African American controls, SCD patients in baseline heath, and SCD patients during acute pain. Plasma samples were assayed in duplicate for SP levels using competitive enzyme-linked immunosorbent assay (Cayman Chemical).  Independent samples student’s t-test was used to compare SP levels between: 1) SCD patients in baseline health and controls and 2) SCD patients in baseline health and those in acute pain. Multivariable linear regression analysis was used to determine the impact of age and gender on SP levels in the 3 independent groups.

Thirty-five African American controls, 25 SCD patients in baseline health, and 12 SCD patients during acute pain completed the study. Controls were older than SCD patients in baseline health (mean (SD) age 21.5 (12.5) vs. 11.8 (3.8) yrs, p=0.0004) and there was no difference in the mean age of SCD patients in baseline health compared to those in acute pain (11.8 (3.8) vs. 12.1 (3.5) yrs, p=0.81). Gender did not differ between the same comparison groups. Genotypes of SCD groups were: 1) baseline: 68% SS, 24% SC, 8% Sβ+thal and 2) acute pain: 67% SS, 25% SC, 8% SOArab. The mean (SD) SP level of SCD patients in baseline health was significantly higher than controls (32.4 (11.6) vs. 22.9 (7.6) pg/mL, p=0.0009) (Figure 1).  When controls were restricted to patients less than 19 years of age (n=21) to better match SCD patients in baseline health, the significant elevation of SP levels in SCD patients persisted (32.4 (11.5) vs. 21 (7.1) pg/mL, p<0.0001). Mean SP levels were even higher during acute SCD pain compared to SP levels of SCD patients at baseline (78.1 (43.4) vs. 32.4 (11.6) pg/mL, p=0.004) (Figure 1). Age and gender were not associated with differences in SP levels in controls or in SCD patients at baseline or during acute pain.

The elevated SP levels in SCD patients during baseline health compared to controls suggests that SP may be a mediator of or marker for chronic pain sensitization. SP levels further increase during acute pain suggesting that SP may also play a role in mediating or marking an acute pain event. The downstream effects of chronically increased levels of SP on the development and propagation of pain in patients with SCD warrant further investigation. Ultimately, SP could differentiate those at risk for development of chronic pain and could be used as a novel biomarker for acute pain and resolution of a painful event. Eventually, compounds aimed at decreasing SP levels or blocking the SP receptor could be a target for novel treatments for SCD pain.