It has been suggested that Chlamydia pneumoniae may play an etiologic role in the development of Alzheimer’s disease. However, studies using polymerase chain reaction to detect this organism in the brains of patients with Alzheimer’s disease have yielded conflicting results. Antibiotics may have a preventive impact on Alzheimer’s disease by some mechanism other than a direct antibacterial effect (e.g., by interfering with amyloid deposition). Loeb and associates investigated whether a combination of doxycycline and rifampin could slow the progression of Alzheimer’s disease.

Patients with Alzheimer’s disease from three outpatient geriatric clinics and two community clinics were enrolled in this randomized, controlled, multicenter trial. Patients received doxycycline and rifampin or placebo for three months. Patients were examined at baseline and at three, six, and 12 months. The primary outcome measure was cognitive function at six months using a standardized instrument. Secondary outcome measures included cognitive function at 12 months, dysfunctional behavior, performance of activities of daily living, and depression. Patients were tested for C. pneumoniae immunoglobulin at baseline and six months, and side effects were monitored.

Of the 101 eligible patients, 84 could be followed up at six months, and 82 could be followed up at 12 months. Most patients in both groups were taking cholinesterase inhibitors during the trial. There was significantly less worsening in cognitive function at six months in the antibiotic group than in the placebo group. Subgroup analysis of patients with high baseline antibody titers to C. pneumoniae showed little significant difference between the antibiotic and placebo groups at six months. Except for dysfunctional behavior at three months, none of the secondary outcomes differed significantly between groups.

The authors conclude that a three-month course of doxycycline and rifampin results in less worsening in cognitive function at six months in patients with Alzheimer’s disease. This result likely represents a halt in disease progression, an outcome that could delay institutionalization in some patients. The laboratory data do not support benefit resulting from treatment of C. pneumoniae infection. The authors suggest that antibiotics may interfere with amyloid deposition in the brain.