Fesoterodine of formula (I) can be chemically described as 2-[(1R)-3-(diisopropylamino)-1-phenylpropyl]-4-hydroxymethyl)phenyl isobutyrate. 

Fesoterodine is an innovative drug for the treatment of overactive bladder, urinary incontinence and other dysfunctions of the urinary tract.  It is disclosed, inter alia, in EP 1077912 B1, pertaining to novel derivatives of 3,3-diphenylpropylamines.  EP 1230209 B1 discloses stable salts of novel derivatives of 3,3-diphenylpropylamines, including Fesoterodine hydrogen fumarate. 
Overactive bladder (OAB) is an extremely common disorder, affecting 17% of the adult population in major European countries.  OAB can occur at any age and in either gender, although its prevalence is higher in geriatric and female populations. 
OAB is a bladder function disorder resulting in symptoms of urgency, with or without urge incontinence, and usually includes increased urinary frequency and nocturia.  The disorder is due to spastic contractions of the detrusor muscle of the bladder, resulting in sustained high bladder pressure and the urgent need to urinate.  This can be caused by several reasons, such as traumatic or toxic nerve damage (e.g., abdominal trauma, pelvic trauma or surgery, bladder stones, adverse effects of drugs), neurological diseases (e.g., spinal cord lesions, multiple sclerosis, Parkinson's disease, excessive neurotransmitter release in the bladder) or myogenic instability (e.g., bladder hypertrophy caused by outlet obstruction or urinary tract infection). 
In some cases, OAB can be managed without pharmacotherapy, using exercise, pessaries, implants, biofeedback or behavioral therapy.  But in most cases, pharmacotherapy is the better option.  Antimuscarinic agents have been found to be particularly effective for treating OAB.  During normal micturition, acetylcholine released from postganglionic parasympathetic neurons acts on the muscarinic receptors of the detrusor smooth muscle in the bladder to stimulate contractions.  Antimuscarinic agents interfere with this action, thus reducing detrusor contractions.  However, despite the availability of different antimuscarinic drugs, physicians and patients remain dissatisfied with current treatments due to adverse events and/or insufficient efficacy.  Furthermore, as a general matter, it is desirable for pharmaceutical compounds to have as little effect on QT intervals as possible.  In particular, it is desirable that there be no significant QT/QTc interval prolongation.  Therefore, new agents with improved safety and efficacy are needed for a more effective treatment of OAB. 
Fesoterodine is known in the art for its potency in treating urinary incontinence.  However, Fesoterodine may exhibit substantial degradation under stress conditions, e.g., in a humid environment and at increased temperature.  It is believed that hydrolyzation and oxidation are among the major mechanisms resulting in degradation.  Therefore, it would be desirable to develop new pharmaceutical compositions comprising Fesoterodine that are more stable against degradation over an extended period of time even under stress conditions.  To that end, it has now been found, surprisingly, that some pharmaceutical excipients are able to significantly slow down the degradation of Fesoterodine under stress conditions.