Clinical concentrations of thioridazine kill intracellular multidrug-resistant Mycobacterium tuberculosis

Abstract

The phenothiazines chlorpromazine (CPZ) and thioridazine (TZ) have equal in vitro activities against antibiotic-sensitive and -resistant Mycobacterium tuberculosis. These compounds have not been used as anti-M. tuberculosis agents because their in vitro activities take place at concentrations which are beyond those that are clinically achievable. In addition, chronic administration of CPZ produces frequent severe side effects. Because CPZ has been shown to enhance the killing of intracellular M. tuberculosis at concentrations in the medium that are clinically relevant, we have investigated whether TZ, a phenothiazine whose negative side effects are less frequent and serious than those associated with CPZ, kills M. tuberculosis organisms that have been phagocytosed by human macrophages, which have nominal killing activities against these bacteria. Both CPZ and TZ killed intracellular antibiotic-sensitive and -resistant M. tuberculosis organisms when they were used at concentrations in the medium well below those present in the plasma of patients treated with these agents. These concentrations in vitro were not toxic to the macrophage, nor did they affect in vitro cellular immune processes. TZ thus appears to be a serious candidate for the management of a freshly diagnosed infection of pulmonary tuberculosis or as an adjunct to conventional antituberculosis therapy if the patient originates from an area known to have a high prevalence of multidrug-resistant M. tuberculosis isolates. Nevertheless, we must await the outcomes of clinical trials to determine whether TZ itself may be safely and effectively used as an antituberculosis agent.

FIG. 1.

Effects of CPZ and TZ on percent annexin V binding by monocyte subsets of Ficoll preparations of PBMCs. Subpopulations of the Ficoll preparation containing PBMCs from healthy donors were separately stimulated for 3 days with M. tuberculosis (H37Rv [ATCC 27294]) or with concentrations of CPZ and TZ that ranged from 0 to 0.5 mg/liter. After 3 days the cells were harvested and stained with fluorescent antibodies against alpha-beta TCR⁺, gamma-delta TCR⁺, and CD14⁺ macrophages in combination with annexin V. Cells were analyzed with an Ortho absolute flow cytometer. Results are expressed as the mean ± standard deviation percent annexin V binding in lymphocytes. Annexin V binding in excess of 10% is an indication of early apoptosis (32). The highest concentrations of CPZ and TZ that had a nominal effect on the percent annexin V binding were 0.1 mg/liter.

FIG. 2.

Average killing activities of human PBMDMs against M. tuberculosis ATCC 27294 (ATCC), two antibiotic-susceptible strains (strains A and B [TB]), and three MDR M. tuberculosis strains (strains C, D, and E; resistant at least to RIF and INH [MDRTB]). The killing activities against the strains used were not significantly different.

FIG. 3.

Average killing activities of human PBMDMs against three MDR M. tuberculosis strains (resistant to RIF and INH) (MDRTB) and three antibiotic-susceptible M. tuberculosis strains (TB) including strain ATCC 27294 swhen either CPZ (A) or TZ (B) was present in the medium at concentrations of 0.01 and 0.1 mg/liter. The control value is the average numbers of CFU for cultures of all six strains to which no phenothiazine was added, inasmuch as the CFU did not differ appreciably between strains.