A human surfactant peptide-elastase inhibitor construct as a treatment for emphysema

Abstract

Two million Americans suffer from pulmonary emphysema, costing $2.5 billion/year and contributing to 100,000 deaths/year. Emphysema is thought to result from an imbalance between elastase and endogenous inhibitors of elastase, leading to tissue destruction and a loss of alveoli. Decades of research have still not resulted in an effective treatment other than stopping cigarette smoking, a highly addictive behavior. On the basis of our previous work, we hypothesize that small molecule inhibitors of human neutrophil elastase are ineffective because of rapid clearance from the lungs. To develop a long-acting elastase inhibitor with a lung pharmacodynamic profile that has minimal immunogenicity, we covalently linked an elastase inhibitor, similar to a trifluoro inhibitor that was used in clinical trials, to a 25-amino-acid fragment of human surfactant peptide B. We used this construct to prevent human neutrophil elastase-induced emphysema in a rodent model. The elastase inhibitor alone, although in a 70-fold molar excess to elastase in a mixture with <0.6% residual elastase activity, provided no protection from elastase-induced emphysema. Covalently combining an endogenous peptide from the target organ with a synthetic small molecule inhibitor is a unique way of endowing an active compound with the pharmacodynamic profile needed to create in vivo efficacy.

Fig. 1.

Sections of mouse lung tissue from PBS and HNE/PBS groups after 4 wk. At the end of testing, animals were killed, lungs were removed, and tissue samples were taken from the hilar and the central regions of the left lung and from the caudal region of the right lung. Each slide was composed of three separate tissue samples. (A and B) Animals instilled with saline had normal lung structure. (C and D) Animals instilled with HNE had extensive lung damage as seen by the large airspaces.

Fig. 2.

Sections of mouse lung tissue from X0/HNE and SP-B1-25-X2/HNE groups after 4 wk. (A and B) Animals given the potent small molecule inhibitor X0 with HNE developed large airspaces characteristic of emphysema (Fig. 1 C and D). The inhibitor, although having a K_i < 12 nM in vitro, had no efficacy in vivo. (C and D) When the small molecule inhibitor X2 was covalently linked to a surfactant peptide fragment to form the construct SP-B1-25-X2, it lost 50-fold potency in vitro but provided complete protection in vivo even after 4 wk.

Fig. 3.

Proposed SP-B1-25–linked therapeutics for other lung ailments. Polymyxins disrupt Gram-negative bacterial membranes and are highly effective even against multidrug-resistant strains, but they have not been widely used because of toxicity concerns. Covalent linkage of a polymyxin to a surfactant peptide fragment (A) can be used to isolate this compound to the lungs and thus potentially minimize or circumvent systemic toxicity. Activating a purinergic receptor requires ATP or an ATP analog. The purinergic P2Y receptor may be targeted to treat cystic fibrosis, and the P2X receptor may be targeted to treat latent tuberculosis. Covalent linkage of ATP to the surfactant peptide (B) may enhance the efficacy of both protocols. Shutting down the EGF family of pathways by stopping cell-surface EGF ligand shedding is a promising new lung cancer strategy. This can be accomplished by inhibiting the ADAM metalloproteinases, but no inhibitor of this class of enzymes has ever successfully passed clinical trials. A strategy of isolating such an inhibitor to the lungs via surfactant linkage (C) may circumvent the pervasive toxicity problems with these types of compounds. A long-duration, infrequent dosing antihistamine (D) may be created with a surfactant-linked strategy.