Elevated plasma substance P in sickle cell disease and vaso-occlusive crisis

Characteristics and potential biomarkers of adult sickle cell patients with chronic pain

OBJECTIVES

In this study, we investigated the evolution of chronic pain in sickle cell patients (SCD) as an age-dependent phenomenon and studied the frequency of vaso-occlusive episode frequency, opioid use, quantitative sensory testing (QST), and biomarkers of chronic pain (CP).

METHODS

We undertook a cross-sectional study of the evolution of CP in SCD. A total of 72 subjects (age 15-66) were enrolled. VOE frequency, presence of CP hydroxyurea (HU) therapy, opioid use, and laboratory parameters were collected. QST was performed, and plasma tryptase, substance P, and NGF (Nerve Growth Factor) levels were assayed.

RESULTS

There was an age-dependent increase in frequency of CP, VOEs, opioid use, and Von Frey monofilament values. CP patients had significantly higher opioid use (daily morphine equivalents) (52.8 mg vs 6.94 mg, P = .009), suggesting a correlation between opioid use and hyperalgesia. NGF levels were also significantly higher (P = .051). Our results confirm previous observations of an age-dependent increase in the proportion of patients with CP and support the contributing role of mast cell activation and neurogenic inflammation.

CONCLUSIONS

This is the first study of NGF as a possible biomarker of CP in SCD. If confirmed, this could provide a diagnostic marker and therapeutic target for CP in SCD.

Substance P is increased in patients with sickle cell disease and associated with haemolysis and hydroxycarbamide use

Sickle cell disease (SCD) pain transitions from acute to chronic for unknown reasons. Chronic elevation of the pain neurotransmitter substance P (SP) sensitizes pain nociceptors. We evaluated SP levels in controls and SCD patients during baseline and acute pain and investigated associations between SP and age, gender, pain history, haemolysis and hydroxycarbamide (also termed hydroxyurea) use. Plasma SP levels were measured using enzyme-linked immunosorbent assay. Independent samples t-test compared SP levels between: (i) SCD baseline and controls, and (ii) SCD baseline and acute pain. Multivariate linear regression determined associations between SP and age, gender, pain history and hydroxycarbamide use. Spearman correlation determined an association between SP and haemolysis. We enrolled 35 African American controls, 25 SCD baseline and 12 SCD pain patients. SCD patients were 7-19 years old. Mean ± standard deviation SP level (pg/ml) in SCD baseline was higher than controls (32·4 ± 11·6 vs. 22·9 ± 7·6, P = 0·0009). SP in SCD pain was higher than baseline (78·1 ± 43·4 vs. 32·4 ± 11·6, P = 0·004). Haemolysis correlated with increased SP: Hb (r = -0·7, P = 0·0002), reticulocyte count (r = 0·61, P = 0·0016), bilirubin (r = 0·68, P = 0·0216), lactate dehydrogenase (r = 0·62, P = 0·0332), aspartate aminotransferase (r = 0·68, P = 0·003). Patients taking hydroxycarbamide had increased SP (β = 29·2, P = 0·007). SP could be a mediator of or marker for pain sensitization in SCD and a biomarker and/or target for novel pain treatment.