Mark John Field

Identification of the α2-δ-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediating the analgesic actions of pregabalin

Neuropathic pain is a debilitating condition affecting millions of people around the world and is defined as pain that follows a lesion or dysfunction of the nervous system. This type of pain is difficult to treat, but the novel compounds pregabalin (Lyrica) and gabapentin (Neurontin) have proven clinical efficacy. Unlike traditional analgesics such as nonsteroidal antiinflammatory drugs or narcotics, these agents have no frank antiinflammatory actions and no effect on physiological pain. Although extensive preclinical studies have led to a number of suggestions, until recently their mechanism of action has not been clearly defined. Here, we describe studies on the analgesic effects of pregabalin in a mutant mouse containing a single-point mutation within the gene encoding a specific auxiliary subunit protein (α2-δ-1) of voltage-dependent calcium channels. The mice demonstrate normal pain phenotypes and typical responses to other analgesic drugs. We show that the mutation leads to a significant reduction in the binding affinity of pregabalin in the brain and spinal cord and the loss of its analgesic efficacy. These studies show conclusively that the analgesic actions of pregabalin are mediated through the α2-δ-1 subunit of voltage-gated calcium channels and establish this subunit as a therapeutic target for pain control.

The monosodium iodoacetate model of osteoarthritis: a model of chronic nociceptive pain in rats?

Osteoarthritis (OA) is a widespread condition affecting the elderly population. One of the most prominent features but least studied symptoms is chronic pain associated with OA. The study objective was to determine pain endpoints in rats with monosodium iodoacetate (MIA) induced OA, and to investigate the efficacy of common nociceptive agents. Sprague-Dawley rats received an intraarticular injection of either 25 microl 80 mg/ml MIA or 25 microl 0.9% sterile saline into the right knee joint. Changes in von Frey thresholds and latencies to stroking with a cotton bud (punctate and dynamic allodynia, respectively) were measured pre- and for up to 10 weeks post-intraarticular injection. Changes in hind paw weight distribution were also determined. Both punctate allodynia and a weight bearing deficit were observed in MIA-treated rats for up to 10 weeks. Interestingly, dynamic allodynia was not detected at any time point tested. Morphine (0.3-3 mg/kg, s.c.) and tramadol (3-100 mg/kg, p.o.) dose-dependently inhibited punctate allodynia and partially reversed weight bearing deficit. In conclusion, the MIA model of OA is reproducible and mimics OA pain in humans. Analgesic drug studies indicate this model may be useful for investigating chronic nociceptive pain.

Pregabalin may represent a novel class of anxiolytic agents with a broad spectrum of activity.

The present study examines the effect of pregabalin (previously S‐Isobutylgaba and CI‐1008) in two distinct rat models of anxiety. Pregabalin binds with high affinity and selectivity to the α2δ subunit of voltage dependent calcium channels (VDCC). Its corresponding R‐enantiomer (R‐isobutylgaba) is approximately 10 fold weaker. Pregabalin dose‐dependently induced anxiolytic‐like effects in both the rat conflict test and elevated X‐maze with respective minimum effective doses (MED) of 3 and 10 mg kg−1. In contrast, R‐isobutylgaba only showed activity at the highest dose of 100 mg kg−1 in the conflict test. These data indicate that pregabalin may possess clinical utility as a novel anxiolytic agent and demonstrates the importance of the α2δ subunit of VDCC in the mediation of anxiety related behaviours.

Synthesis and in vivo evaluation of bicyclic gababutins

Synthesis of a number of bicyclic five-membered ring derivatives of gabapentin led to the identification of two compounds, (-)-(11A) and (20A) which both had an excellent level of potency against alpha(2)delta and were profiled in an in vivo model of neuropathic pain.

[3H] pregabalin binding is increased in ipsilateral dorsal horn following chronic constriction injury

Pregabalin, a 3-substituted analogue of gamma-amino butyric acid has recently been approved for treatment of neuropathic pain. We have investigated the anatomical binding profile of [(3)H] pregabalin following chronic constriction injury (CCI) and compared this with alpha 2 delta 1 subunit expression using in situ hybridisation. We report here that the intensity and distribution pattern of [(3)H] pregabalin binding is altered in the ipsilateral dorsal horn following CCI and this is associated with a corresponding increase in alpha 2 delta 1 mRNA in the ipsilateral dorsal root ganglion (DRG). It is likely that increased DRG mRNA production leads to increased alpha 2 delta 1 protein production and subsequent transport by primary afferents to the dorsal horn. The increased expression of calcium channel subunits and protein in central terminals is interesting, given that abnormal activity within sensory nerves is likely to significantly contribute to the symptomatology of neuropathic pain. The upregulation of pregabalin binding sites in sensory nerve terminals may occur as part of the response to nerve damage in neuropathic pain patients, and therefore, preferential actions of pregabalin at these sites may contribute to its mechanism of action in man.

Pharmacokinetic–Pharmacodynamic Analysis of the Static Allodynia Response to Pregabalin and Sildenafil in a Rat Model of Neuropathic Pain

The objective of this study was to develop a pharmacokinetic–pharmacodynamic (PK-PD) model of the static allodynia response to pregabalin with and without sildenafil in a chronic constriction injury model of neuropathic pain. Six treatment groups were evaluated every 30 min for 6 h. Rats were treated with either 1) a saline infusion; 2) a 2-h pregabalin infusion at 4 mg·kg−1·h−1; 3) a 2-h pregabalin infusion at 10 mg·kg−1·h−1; 4) a 2.2-mg loading dose + 12 mg·kg−1·min−1 infusion of sildenafil; 5) a 2-h pregabalin infusion at 1.6 mg·kg−1·h−1 with sildenafil; and 6) a 2-h infusion of pregabalin at 4 mg·kg−1·h with sildenafil. The static allodynia endpoint was modeled by using three population PD approaches: 1) the behavior of the injured paw using a three-category ordinal logistic regression model; 2) paw withdrawal threshold (PWT) (g) between the injured and uninjured paw using the Hill equation with a baseline function; and 3) the baseline normalized difference in PWT between the injured and uninjured paw. The categorical model showed a significant shift in the concentration–response relationship of pregabalin to lower concentrations with concomitant sildenafil. Likewise, the continuous PK-PD models demonstrated a reduction in the EC50 of pregabalin necessary for PD response in the presence of sildenafil. The difference-transformed PD model resulted in a 54.4% (42.3–66.9%) decrease in EC50, whereas the percentage-transformed PD model demonstrated a 53.5% (42.7–64.3%) shift. It is concluded from these studies that there is a synergistic PD interaction between pregabalin and sildenafil.

Synthesis and in vivo evaluation of 3,4-disubstituted gababutins

A range of 3,4-alkylated five-membered ring derivatives of gabapentin were synthesised. One compound (21) had an excellent level of potency against alpha(2)delta and was profiled in in vivo models of pain and anxiety.

Synthesis and in vivo evaluation of 3-substituted gababutins.

A range of 3-alkylated five-membered ring derivatives of Gabapentin were synthesized and several were found to have good levels of potency against the alpha2delta calcium subunit of a voltage-gated calcium channel. Two compounds were profiled in in vivo models of pain and anxiety.