Spasmolytic activity of hyoscine butylbromide in gastrointestinal and non-gastrointestinal tissues after oral and intravenous administrations.

Background and Purpose: Hyoscine butylbromide (HBB) has a low oral (PO) bioavailability. Further, limited data on its activity on non-gastrointestinal (GI) smooth muscle spasms after oral dosing are available, meaning its effect outside the GI tract has been questioned. This pharmacokinetic/pharmacodynamic (PK/PD) study, conducted using female rats, aims to cover this gap. Experimental Approach: PK study: HBB and atropine (comparator) were administered PO and IV to rats, and concentrations in plasma, and colon, uterus and urinary bladder (CUB) were measured. PD study-1: Concentration–response (C-R) curves of HBB and atropine (10 -9 –10 -4 M) were obtained for carbachol (CCH)-induced (10 -5 M) pre-contracted CUB; PD study-2: CUB were pre-incubated with HBB and atropine at maximum concentrations (Cmax) from PK study, and CCH C-R curves (10 -9 –10 -4 M) were obtained; PD study-3: HBB and atropine were administered PO and IV to rats as for PK study, CUB were collected at 0.5 h (IV) and 4 h (PO), and CCH C-R curves (10 -9 –10 -4 M) were obtained. Key Results: PO HBB showed higher Cmax in CUB (192.5, 3.70 and 1.85 ng∙g -1 , respectively) than plasma (0.008 ng∙mL -1 ). HBB and atropine reduced (concentration dependently) CCH-induced contractions in CUB. PO HBB showed highest spasmolytic activity in colon (40%), followed by uterus (30%) and urinary bladder (10%). Conclusion and Implications: This is the first comparative study investigating PO and IV HBB and atropine in GI and non-GI tissues. Despite low bioavailability, PO HBB accumulates and exerts spasmolytic effects outside the GI tract.