Continuous intrathecal (IT) morphine infusion is used in chronic pain patients. A limitation is that morphine results in an inflammatory cell mass (granuloma) arising from the meninges. We recapitulated these observations in a canine model wherein lumbar CSF concentrations leading to granulomas were comparable to those observed in humans. In the last two funding cycles of this grant, we showed that granulomas are induced in a concentration dependent fashion by morphine and other opiates and not at all by fentanyl or alfentanil. We hypothesized that the granuloma arises from the degranulation of meningeal mast cells (MMC). Thus: i) opiates that degranulate MMCs ex vivo and cutaneous flare after subcutaneous (SQ) delivery produce a granuloma; ii) the meningeal/subcutaneous degranulation/flare and the granuloma are blocked by the MC stabilizer cromolyn, but not by opiate antagonism. The origin of this opiate receptor-independent MC degranulation is hypothesized to reflect the cationic properties of the nonpeptide and peptide opioid ligands acting to degranulate mast cells through G protein coupled receptor families, such as the Mas-related gene like receptors (MrgX). These observations jointly lead to an elaboration of hypotheses to characterize and avoid the granuloma. Hypothesis 1: Degranulation of mast cells by opioid agents is independent of opiate receptor activation but potentially depend upon receptors activated by cationic charge (MrgX; HFPR). Hypothesis 2. The opiate granuloma is independent of opiate receptor activation but will covary with ability of the nonpeptide and peptide opioid ligands to degranulate mast cells. In hypothesis 1, we will examine concentration dependent effects of opioid (DAMGO, TAPP, DALGA, and DMT- DALGA) and nonopioid peptides (ziconotide) and non-peptides (e.g. baclofen, clonidine, neostigmine) on: flare in the dog, mast cell degranulation in human primary mast cell cultures and on murine primary cell cultures. Using the human mast cell cultures, we will examine the role of MgrX-r using shRNA to reduce that protein expression and define the role of that cationic receptor on mast cell degranulation. In hypothesis 2, we will i) undertake dose response curves in dogs with IT infusion of the above mentioned mu opioid peptides to define the just maximally effective analgesic dose: JMEAD and the maximum tolerable (e.g. acute side effect limited) dose (MTD), ii) define intrathecal PK of selected agent and iii) determine if infusion of the maximum equi-effective (analgesic) doses of these peptides lead to a granuloma.
Spinal opiate infusion is a recognized chronic pain therapy and spinal masses (granulomas) are a recognized risk of its use. Our work has implicated meningeal mast cells and the present activity will define the ability of agents with high opioid potency and a low propensity for mast cell degranulation to reduce the likelihood of spinal granuloma formation at analgesic doses. We believe these findings and proposed studies have great clinical relevance to developing safer agents for spinal therapy.
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