Synthesis of 99m Tc-oxybutynin for M 3 -receptor-mediated imaging of urinary bladder M. E. Moustapha • M. A. Motaleb • I. T. Ibrahim Received: 27 April 2010 / Published online: 19 August 2010 Ó Akade ´miai Kiado ´, Budapest, Hungary 2010 Abstract Radiolabeling of oxybutynin, a muscarinic acetylcholine (mACh) receptor antagonist agent with 99m Tc is of considerable interest for imaging of urinary bladder. This study is aimed to optimize radiolabeling yield of oxybutynin with 99m Tc using SnCl 2 Á2H 2 O as a reducing agent with respect to factors that affect the reaction con- ditions such as oxybutynin amount, stannous chloride amount, reaction time and pH of the reaction mixture. In vitro stability of the radiolabeled complex was checked and it was found to be stable for up to 8 h. 99m Tc-oxybutynin was injected via subcutaneous and intravenous adminis- tration routes into normal Sprague–Dawley rats. Biodis- tribution studies have revealed that 99m Tc-oxybutynin exhibits high affinity and specificity for the muscarinic M 3 subtype located on the smooth muscle of urinary bladder relative to the M 1 and M 2 subtypes of the G protein cou- pled receptor (GPCR) superfamily. In vivo uptake of sub- cutaneous 99m Tc-oxybutynin in urinary bladder was 19.6 ± 0.42% ID at 0.5 h, whereas in intravenous admin- istration route the accumulation in the urinary bladder was found to be 9.4 ± 0.31% ID at 0.5 h post injection. Administration of cold oxybutynin effectively blocked urinary bladder uptake and further confirms the high specificity of this complex for the M 3 receptor. Keywords Oxybutynin Á Imaging Á Urinary bladder Á Radiolabeling Á M 3 receptor Á Radiopharmaceuticals Introduction Oxybutynin, 4-Diethylaminobut-2-ynyl-2-cyclohexyl-2- hydroxy-2-phenyl-ethanoate (Fig. 1), is a muscarinic ace- tylcholine (mACh) receptor antagonist agent developed as an antispasmodic, anticholinergic drug for the treatment of symptoms of overactive bladder (OAB) such as urge urinary incontinence [1]. Consequently, relaxes bladder smooth muscle in patients with conditions characterized by involuntary bladder contractions [2, 3]. The urinary bladder is under the control of the parasympathetic nervous system, which classically releases acetylcholine (ACh). ACh in turn acts via muscarinic receptors located on the detrusor smooth muscle of the urinary bladder to evoke contraction of the bladder smooth muscle and so voiding of urine [4]. The muscarinic acetylcholine (mACh) receptor family consists of five members and belongs to the G protein- coupled receptor (GPCR) superfamily [5, 6]. The five mACh receptor subtypes have been identified and classified (M 1 –M 5 ) based on genetic and pharmacological charac- teristics [7–9] and genetic/gene knockout [10–12]. Although both M 2 and M 3 subtypes coexist in smooth muscle, muscarinic M 3 receptors are primarily responsible for the contraction of most smooth muscle, including the urinary bladder [13]. The M 2 receptors may have a role in mediating indirect contractions and/or inhibition of detru- sor relaxation [14]. In vitro radioligand studies with human recombinant muscarinic subtypes have revealed that oxy- butynin exhibits high affinity and specificity for the mus- carinic M 3 subtype located on the smooth muscle of urinary bladder relative to the M 1 and M 2 subtypes [15, 16]. On the basis of these results, mACh receptor antagonists with a higher affinity for M 3 than M 2 subtypes should be more beneficial in the treatment of overactive bladder. Different reviews have explored the binding of M. E. Moustapha Á M. A. Motaleb Á I. T. Ibrahim Labeled Compound Department, Hot Lab. Centre, Atomic Energy Authority, P.O. Box 13759, Cairo, Egypt M. E. Moustapha (&) Chemistry Department Faculty of Science, Benha University, Benha, Egypt e-mail: memoustapha@gmail.com 123 J Radioanal Nucl Chem (2011) 287:35–40 DOI 10.1007/s10967-010-0794-z