Intracellular activity of clinical concentrations of phenothiazines including thioridiazine against phagocytosed Staphylococcus aureus

Abstract

The effect of thioridazine (TZ) was studied on the killing activity of human peripheral blood monocyte derived macrophages (HPBMDM) and of human macrophage cell line THP-1 at extracellular concentrations below those achievable clinically. These macrophages have nominal killing activity against bacteria and therefore, would not influence any activity that the compounds may have against intracellular localised Staphylococcus aureus. The results indicated that whereas TZ has an in vitro minimum inhibitory concentration (MIC) against the strains of S. aureus of 18, 0.1 mg/l of TZ in the medium completely inhibits the growth of S. aureus that has been phagocytosed by macrophages. The latter concentration was non-toxic to macrophages, did not cause cellular expression of activation marker CD69 nor induction of CD3+ T cell production of IFN-γ, but blocked cellular proliferation and down-regulated the production of T cell-derived cytokines (IFN-γ, IL-5). These results suggest that TZ induces intracellular bactericidal activities independent of the capacity to generate Type 1 responses against S. aureus.

Introduction

Recurrent Staphylococcus aureus infections are difficult to manage due to the incomplete killing activity of the macrophage [1], [2], [3] or due to the limited accessibility of the antibiotic to the organism sequestered within the macrophage subsequent to its phagocytosis [4], [5], [6]. When the organism is resistant to methicillin, a problem that has become acute in a nosocomial setting, the use of toxic antibiotics is often the only alternative available [7]. The neuroleptic phenothiazine, chlorpromazine (CPZ) introduced by Rhone–Poulenc in the early 1950s as well as other phenothiazines such as thioridazine (TZ), promazine (PZ), promethazine (PMZ) and desipramine (DES), have in vitro activity against a variety of bacteria at concentrations beyond clinical reach [8], [9]. However, CPZ is concentrated in excess of 100-fold by tissues and organs rich in macrophages [10], [11] and in vitro concentrations that are within clinical reach are effective against phagocytosed Mycobacterium tuberculosis [11]. The neuroleptic TZ, a phenothiazine devoid of the toxic properties of CPZ, is as effective as CPZ in vitro, against all strains of M. tuberculosis regardless of their antibiotic susceptibility status [12], [13]. As TZ is also effective against S. aureus [14], we questioned whether TZ has activity against S. aureus after the organism has been phagocytosed by human peripheral blood monocyte-derived macrophages (HPBMDM) or by the macrophage cell line THP-1, both of which have nominal killing activity against this bacterium [15]. The concentrations of CPZ and TZ in the medium in this study are well below the in vitro minimum inhibitory concentrations (MIC) against S. aureus and are achievable clinically. Whether these concentrations affect the phagocytic process, are toxic to the macrophage or affect functional activities of these cells, as well as whether intracellular killing of the organism is associated with or is independent of cellular immune responses derived from T-cells or macrophages, were investigated.