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Treatment of Overactive Bladder: A Model Comparing Extended-release Formulations of Tolterodine and Oxybutynin
Eleanor M. Perfetto, PhD; Prasun Subedi, MS; and Zhanna Jumadilova, MD, MBA

Treatment of Overactive Bladder: A Model Comparing Extended-release Formulations of Tolterodine and Oxybutynin

Eleanor M. Perfetto, PhD; Prasun Subedi, MS; and Zhanna Jumadilova, MD, MBA

Objective: The objective of this study was to compare 1-year total healthcare costs for patients with overactive bladder (OAB) initiating treatment with extended-release formulations of tolterodine and oxybutynin: tolterodine tartrate extended-release capsules (tolterodine ER) versus extended-release oxybutynin chloride (oxybutynin ER).

Methods: A model was developed from the payer perspective using data from the PharMetrics Patient-Centric database. Monthly discontinuation rates were derived from a cohort of newly treated patients with OAB (tolterodine ER, n = 15 394 or oxybutynin ER, n = 7934). All were assumed to be receiving therapy for at least 1 month. Medical management costs were based on reimbursement for all services for a matched cohort of patients taking tolterodine ER and oxybutynin ER. Medical management costs for those discontinuing therapy were based on patients receiving OAB care without pharmacotherapy (n = 29 992). Drug costs were from AnalySource (December 2004).

Results: After the 11-month follow-up period, 21% of patients taking tolterodine ER and 15% of patients taking oxybutynin ER remained on original therapy. One-year average total costs per patient for those started on tolterodine ER were $8876 and $9080 for oxybutynin ER, a difference of $204 per year. Sensitivity analyses indicated results were robust to changes in drug cost and probability of discontinuation. When discontinuation rates were held equal, cost differences continued to favor tolterodine ER (21%, $272/yr; 15%, $233/yr).

Conclusion: Those taking tolterodine ER had lower monthly drug and medical management costs. This resulted in a total average annual cost savings of $204 per patient for those started on tolterodine ER. At the end of 1 year, patients with OAB were more likely to remain on original drug treatment taking tolterodine ER versus oxybutynin ER.

(Am J Manag Care. 2005;11:S150-S157)


Overactive bladder (OAB) is defined by the presence of urgency (with or without urge incontinence), usually with frequency and nocturia, and the absence of pathologic or metabolic conditions that might explain these symptoms. Recent figures from the National Overactive BLadder Evaluation (NOBLE) program show that the overall prevalence of OAB is 16.5% in the United States—representing more than 34 million men and women.1,2 Studies indicate that the prevalence of OAB in European nations may be similar: a survey of 16 776 subjects aged 40 years or older found the prevalence of OAB to be 16.6% across 6 European countries.3

In addition to the intangible costs (ie, the emotional and physical toll that OAB takes on individuals), the economic burden of this disease is considerable. The direct and indirect costs of OAB in 2000 were estimated to total $12.02 billion in the United States.2 A recent study analyzed US claims data from privately insured patients 18 to 64 years of age. Compared with patients without OAB, patients with OAB had significantly higher average total claim costs ($1767 vs $5018, 1995 dollars, respectively; P = .0001), a significantly higher probability of hospital admission (7.0% vs 20.6%, respectively; P = .0001), and a significantly higher average length of stay once admitted (4.8 vs 6.8 days, respectively; P = .0001) for the study period.4 Other studies indicate that costs can be decreased when OAB is properly diagnosed and treated.5,6

Clinicians and payers need comparative information to support their clinical and formulary decision making. To respond to this need, a number of pharmacoeconomic models have been used to illustrate comparisons among antimuscarinic treatments for OAB. Earlier studies have focused on immediate-release formulations; and some compared immediate-with extended-release products.7-13

Several recently published models have compared the extended-release formulations with each other because these treatments are now more commonly used. In one such model incorporating more than 30 resource and cost inputs, the average 3-month overall treatment costs for tolterodine tartrate extended-release capsules (tolterodine ER) therapy were 6% less than that of oxybutynin chloride extended release (oxybutynin ER) therapy ($1207 vs $1283) for OAB patients initiating long-acting drug therapy in a primary care setting.14 This model included estimates of incontinence pad use and lost productivity based on published data.

A cost-effectiveness model developed by Hughes and Dubois compared the extendedrelease forms of tolterodine and oxybutynin.15 This study included patient-level data on incontinence episodes and pad use from clinical trials and from the literature, incorporating persistence and adverse effects (eg, dry mouth or central nervous system side effects). Both extended-release formulations were found to be cost effective, and their rank ordering in the results were influenced by "uncertainty surrounding the health and cost consequences of early discontinuations."15

Economic models of OAB therapy are often developed so managed care organizations (MCOs) can compare treatments to assist in formulary decision making and input their own plan-specific information into the model. However, in the majority of circumstances, most MCOs do not have the specific clinical information needed for the model inputs. For example, the number of incontinence episodes experienced by a patient is often an important input in OAB economic models, but the majority of MCOs do not have systems in place to accurately track such variables. Moreover, most plans do not cover certain costs, such as the cost of incontinence pads, diapers, or the cost of laundry. Thus, the costs associated with the utilization of such materials would likely not factor into their coverage decision making. Thus, it is unlikely that a typical insurer or MCO has the clinical inputs required to customize these models, nor would they be greatly concerned with some of these costs.

There are, however, a number of potential model inputs that insurers can track easily within their population and that have an impact on reimbursement. For example, payers know their drug costs for pharmacy claims, and most can track how long an enrollee is persistent with therapy, or if the patient discontinues the drug. An OAB economic model that incorporates these types of inputs is, therefore, not only more relevant to the insurer in terms of reimbursement costs, but is more easily customizable and can be used to more accurately reflect the unique experiences of a given insurer's OAB population. The objective of this study was to compare tolterodine ER and oxybutynin ER in a cost-minimization model that is not only user friendly, but also includes data readily available to most payers as inputs into the model.

Methods

Model Design. A decision-analysis model was developed using Microsoft Excel comparing the 12-month economic impact of treating OAB. The diagram in the Figure depicts a schematic of the decision-analysis model used. Because they are the most widely prescribed drugs for OAB, the comparison is limited to the long-acting forms of tolterodine and oxybutynin (tolterodine ER and oxybutynin ER). The model takes the payer's perspective and a cost-minimization approach; treatment outcomes are assumed to be the same for patients who persist with therapy. Because the frequency of therapy switching has been found to be relatively low (~6%),16 it was assumed that when treatment was discontinued, patients were medically managed without pharmacotherapy. Discontinuation of initial therapy (which also includes patients who switched from therapy) was accounted for on a monthly basis.

Figure Data Sources

Medication costs and medical management costs are included. Medication costs are calculated using the average wholesale price (AWP). The AWP for both drugs was obtained in December of 2004 from Analy-Source, an online vendor of drug pricing data.17 The cost for tolterodine ER was based on Detrol LA® 4 mg and was assumed to be $3.31 per dose, or $99.30 per month. Oxybutynin ER cost was based on Ditropan XL® 10 mg and was assumed to be $3.38 per dose, or $101.40 per month.

In addition to the costs associated with OAB pharmacotherapy, the model also incorporates costs for medical management. These include the costs of non-OAB-related pharmacy services, as well as the cost of inpatient, outpatient, and emergency services used by OAB patients over the course of 1 year. Both medical management costs and the rates of OAB therapy discontinuation were obtained from the PharMetrics Patient-Centric database, which is composed of fully adjudicated medical and pharmacy claims for more than 55 million covered lives from more than 75 health plans across the United States.18 The PharMetrics database includes both inpatient and outpatient diagnoses (in International Classification of Diseases, Ninth Revision, Clinical Modification format) and procedures (in Current Procedural Terminology 4 and Healthcare Common Procedures Coding System formats), as well as both standard and mailorder prescription records. Data on prescription records include the National Drug Code, as well as "days supplied" and "quantity dispensed" fields.

Records in the PharMetrics database are representative of the national managed care population based on a variety of patient and health plan demographic measures, including geographic region, age, sex, and plan type. The data are also longitudinal, with an average member enrollment time of 2 years. Only health plans submitting data for all members are included in the database, ensuring complete data capture and unbiased samples.

The sample population for this study consisted of all patients with either a new diagnosis of OAB or new use of 1 of the medications of interest (tolterodine or oxybutynin in any form) between January 1, 2001, and December 31, 2002. The date of first OAB diagnosis or medication use served as the patient's index date. All patients were required to have pre-index and follow-up periods of 12 months each in relation to this date. Claims for selected patients were then accumulated spanning the period from January 1, 2000, to December 31, 2003. It was found that 80% of all pharmacologically treated patients with OAB were using tolterodine ER or oxybutynin ER, with the remaining 20% taking one of the immediate-release formulations.

A propensity score was calculated for each patient with OAB. The estimated propensity scores were intended to represent a particular patient's probability for receiving a given treatment option and were calculated by summing coefficient values for a list of potential confounding variables. Use of these scores confers the advantage of having a single estimate available to adjust for confounding in a multivariate analysis. In this case, the outcome of interest was the propensity for treatment with any of the selected medications for OAB.

Patient Cohorts

Using these data, 3 cohorts were created for deriving the model inputs:

Cohort 1. All patients taking tolterodine ER and oxybutynin ER, selected as described above, were included in this cohort. The cohort included 15 394 treated with tolterodine ER and 7934 patients treated with oxybutynin ER. Using these data, monthly discontinuation rates were calculated for each drug group (Table 1).

Figure

Cohort 2. From among those in Cohort 1, the tolterodine ER and oxybutynin ER patients were matched based on the estimated propensity score for OAB pharmacotherapy to create a matched cohort. After matching, a cohort of 7257 tolterodine ER and 7257 oxybutynin patients resulted. The average annual medical costs for each treatment group were then calculated. Costs were derived from the amounts paid by the insurer for services rendered. These costs were included as the medical management costs for these patients, excluding the OAB drug treatment costs (Table 2).

Figure

Cohort 3. All patients who were treated with any OAB medication were matched based on the estimated propensity score to a control group of patients with OAB who were not receiving any OAB pharmacotherapy. After matching, there were 29 992 patients in the OAB-drug-treated group and 29 992 patients in the medically managed group (no pharmacotherapy). The individuals not receiving OAB pharmacotherapy were considered to be a "medically managed" group, and the average cost of medical treatment for this group was used to account for monthly medical costs for patients who discontinued treatment with a study drug for the remainder of the study period (Table 2).

 
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