ORIGINAL INVESTIGATION Comparison of Central Nervous System Adverse Effects of Amantadine and Rimantadine Used as Sequential Prophylaxis of Influenza A in Elderly Nursing Home Patients Linda A. Keyser, PharmD; Margaret Karl, RPh; Anne N. Nafziger, MD, MHS; Joseph S. Bertino, Jr, PharmD Background: Amantadine hydrochloride and rimanta- compared with 3 patients (1.9%) when rimantadine was dine hydrochloride are recommended by the Centers for given (P,.01). Drug use was discontinued due to ad- Disease Control and Prevention for prophylaxis of in- verse events in 17.3% (n=27) of the amantadine courses fluenza A. While data suggest that rimantadine is better and 1.9% (n=3) of the rimantadine courses (P,.001). tolerated, there are no data examining the rate of ad- Confusion was the most frequently observed adverse event verse reactions in elderly patients who receive amanta- (amantadine, 10.6%; rimantadine, 0.6%; P,.001). Mul- dine vs rimantadine. Our objective was to assess the ad- tivariate logistic regression analysis showed that signifi- verse reaction rate in elderly nursing home patients cant risk factors for central nervous system adverse events receiving sequential amantadine and rimantadine for in- included male sex (odds ratio, 3.65), reduced calcu- fluenza A prophylaxis. lated creatinine clearance (odds ratio, 1.78), and use of amantadine (odds ratio, 12.73). Methods: Data were collected in 156 nursing home pa- tients (70% women; mean±SD age, 83.7±10.1 years) in Conclusions: Amantadine use was associated with a sig- a single care setting who received sequential therapy with nificantly higher incidence of central nervous system ad- amantadine and rimantadine during the 1997-1998 in- verse events than rimantadine use in this elderly popu- fluenza season. Patients were assessed for central ner- lation receiving influenza prophylaxis. In addition, the vous system adverse effects and therapy discontinua- discontinuation rate of amantadine was significantly tion occurring with each agent. higher than that with rimantadine. Results: Twenty-nine (18.6%) of the 156 patients ex- perienced an adverse effect when receiving amantadine Arch Intern Med. 2000;160:1485-1488 NFLUENZA A is a common cause than rimantadine.1-3 To date, there are no of serious respiratory illness in the data in elderly nursing home patients on elderly population.1 Among per- the comparative toxicity of these agents. sons aged 65 years and older, Since the cost of amantadine is signifi- rates of hospitalization for influ- cantly less than that of rimantadine, enza range from 200 to more than 1000 amantadine continues to have high I 1 per 100000 population. Immunization usage in the nursing home setting. If in with influenza vaccine has been shown to fact a high incidence of AEs occurs with be 50% to 60% effective in preventing hos- amantadine use, this could lead to pitalization and pneumonia, 80% effec- excessive discontinuation rates and pos- tive in preventing death, and 30% to 40% sibly put patients at risk of developing From the University of effective in preventing disease in elderly influenza A. Colorado School of Pharmacy, populations. However, because of the lim- During the 1997-1998 influenza sea- Denver (Dr Keyser), and the ited effectiveness of the vaccine, chemo- son, residents at a rural Upstate New York Department of Pharmacy prophylaxis is often indicated during sus- nursing home received amantadine and Services (Ms Karl and pected or confirmed outbreaks.1 rimantadine sequentially during a sus- Dr Bertino), the Department of Two antiviral agents are approved pected and a documented influenza A out- Medicine (Drs Nafziger and for influenza prophylaxis, amantadine break. For each agent, we assessed the in- Bertino), The Research Institute (Drs Nafziger and Bertino), hydrochloride and rimantadine hydro- cidence of CNS AEs in this cohort of and the Clinical Pharmacology chloride. While both agents are effective patients and attempted to identify risk fac- Research Center (Drs Nafziger in the prophylaxis of influenza A, aman- tors for AEs. In addition, we evaluated the and Bertino), Bassett tadine appears to cause more central ner- rate of discontinuation of drug second- Healthcare, Cooperstown, NY. vous system (CNS) adverse effects (AEs) ary to AEs. ARCH INTERN MED/ VOL 160, MAY 22, 2000 WWW.ARCHINTERNMED.COM 1485 ©2000 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/02/2021 Table 1. Dosing Scheme for Amantadine PATIENTS AND METHODS Use in Study Patients Calculated Creatinine Clearance* Maintenance Dosage This was a retrospective cohort study to assess the $0.83 ($50) 100 mg twice a day AEs of antiviral administration given as prophylaxis (Patients ,65 years old) for influenza A. The study population included 167 $0.83 ($50) 100 mg/d residents of an Otsego County, New York, long- (Patients $65 years old) term care facility for the elderly. All residents re- 0.50-0.82 (30-49) 200 mg on day 1 then 100 mg/d ceived influenza vaccine in October to November 0.25-0.49 (15-29) 200 mg on day 1 then 100 mg 1997. In January 1998, an influenza A outbreak oc- every other day curred in the community and several residents of the ,0.25 (,15) 200 mg every 7 d nursing home developed symptoms consistent with (and patients receiving influenza infection. Cultures were obtained; how- hemodialysis) ever, none were positive. A planned 24-day course of amantadine for prophylaxis was begun for all resi- *Given in units of milliliters per second per 1.73 square meters (milliliters per minute per 1.73 square meters). dents. Dosage adjustment (Table 1) was done us- ing creatinine clearance (ClCr) calculated with the following equation4: RESULTS ClCr=[(140−Age)(Ideal Body Weight)]/(723Scr) (multiplied by 0.85 for women), Of a total of 167 patients at the nursing facility, 156 pa- tients received amantadine therapy followed by riman- where ClCr is given in milliliters per minute per tadine therapy. Eleven patients died, either during aman- 1.73 square meters, Scr indicates serum creatinine in tadine therapy or between the time of drug therapies. Of milligrams per deciliter, and weight in kilograms. In the patients who died during amantadine therapy, none February 1998, approximately 1 week after comple- tion of the amantadine course, a confirmed influenza had positive cultures for influenza A. Table 2 gives the A (via positive cultures) outbreak occurred in the demographic data on the 156 patients included in the nursing home. At this time, the New York State analysis. Department of Health mandated that all patients The average duration of therapy in all patients was receive rimantadine hydrochloride, 100 mg/d, for 28 20.6 ± 6.7 days for amantadine and 26.3 ± 5.3 days for days. rimantadine (P=.21). Duration of therapy in patients who Nursing staff were in-serviced by a pharmacist be- experienced CNS toxic effects was 10.9 ± 4.4 days in the fore the initiation of antiviral therapy. The regular nurs- amantadine group and 16.3 ± 10.6 in the rimantadine ing staff at the facility collected detailed information group. For patients who did not have toxic reactions, du- on the clinical course of each patient during both the ration of therapy for amantadine was 22.6 ± 5.1 days and amantadine and rimantadine treatment periods. All CNS AEs (including new-onset agitation, aggression, con- for rimantadine, 26.5 ± 5 days (P#.05 vs patients with fusion, decreased coordination, hallucinations, leth- toxic effects for each drug). argy, paranoia, seizures, and tremors) were carefully Table 3 outlines the concurrent CNS conditions documented. Patients were assessed daily for CNS AEs and CNS active medications for the 156 patients. One hun- (while receiving therapy) with the information (pres- dred five (67%) of the residents were receiving 1 CNS ence or absence of CNS AEs and a description of such) active medication. Forty-one (26%) of the residents were recorded in the patient chart. taking multiple CNS medications. No additional CNS ac- Data collected for this study included age, sex, tive medications were noted to be used during the study weight, height, current CNS disease states (eg, his- period (ie, opioids, antihistamines, antiemetics). There tory of seizures, stroke, and transient ischemic at- was no change in concurrent CNS conditions or use of tack), dose of amantadine and rimantadine, dura- tion of antiviral therapy, concurrent CNS active CNS active medications during the amantadine or riman- medications, reason for drug discontinuation be- tadine treatment periods. fore the planned course of therapy was completed, Table 4 shows the comparative CNS AEs seen dur- and all noted CNS AEs. ing amantadine and rimantadine therapy. Significant dif- Only patients who received both of the antiviral ferences were seen in the total number of patients with agents were included in the analysis. Data were ana- CNS-related AEs and those requiring drug discontinua- lyzed using SAS version 6.08 software.5 The McNe- tion in the amantadine vs the rimantadine treatment pe- mar test was used for categorical variables. The paired riods (P,.001). Significant differences were seen in rates t test was used to examine duration of therapy. Mul- of confusion (P,.001) during the treatment periods. No tivariate stepwise logistic regression analysis was used falls were reported in either group during either treat- (using the demographic, underlying disease, and con- current CNS drug therapy data) to evaluate risk fac- ment phase. In addition, no hospitalizations or aspira- tors for AEs and for early discontinuation of therapy. tion pneumonia episodes resulted from CNS AEs. When significant risk factors were found by logistic Table 5 gives the results of the multivariate analy- regression analysis, interaction terms were exam- ses of risk factors for CNS AEs and for drug discontinu- ined. P#.05 was considered significant.