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Background: Histamine is a critical mediator of immediate hypersensitivity reactions. Sensory neuropeptides, such as substance P (SP), may also contribute to acute inflammatory responses. A compound which antagonizes both H₁ and NK-1 receptors, such as MDL 108,207DA, may present a significant therapeutic advantage over pure antihistamines. Methods: The binding affinity of MDL 108,207DA for H₁ and NK-1 receptors was evaluated and its potency of antagonism evaluated in vitro. The in vivo antagonism of SP- or histamine-induced microvascular leakage in guinea pig airways was examined. A role for these mediators in antigen-induced microvascular leakage in ovalbumin-sensitized guinea pig airways was examined using MDL 108,207DA as well as the NK-1-selective antagonist FK888 and the H₁-selective antagonist pyrilamine alone or in combination. Results: The affinity of MDL 108,207DA for H₁ and NK-1 receptors is similar to that of receptor-selective antagonists. The compound inhibits both receptors in vitro and in vivo with comparable potencies for each. The efficacy of FK888 in combination with pyrilamine and MDL 108,207DA on antigen-induced microvascular leakage in sensitized guinea pig airways supports a role for both SP and histamine in early allergic responses. Conclusion: The contribution of both SP and histamine to immediate hypersensitivity reactions supports the utility of NK-1 and H₁ receptor antagonist therapy. MDL 108,207DA incorporates both activities into the same compound and, as a result, may be useful in the treatment of allergic diseases.

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