Protective Effect of Subinhibitory Polymyxin B Alone and in Combination with Ampicillin for Overwhelming Haemophilus influenzae Type B Infection in the Infant Rat: Evidence for in Vivo and in Vitro Release of Free Endotoxin after Ampicillin Treatment

Abstract

A B S T R A O . The potential endotoxin modifying effects of subinhibitory doses of polymyxin B were evaluated in an animal model of overwhelming septicemia. Five to six day old Sprague-Dawley rats were infected intraperitoneally with lo6-lo7 cfu of Haemophilus influenzae type b. At 1 2 h after infection, a t which time mortality was 1870, subinhibitory doses of polymyxin b (0.0125 mg/kg x 3 q 3 h) either alone or in combination with 500 mg/kg ampicillin significantly increased survival a t 1 7 and 20 h (p = 0.009, 0.01 and p = 0.003, 0.01) compared to animals treated with 0.5 mg/kg of ampicillin alone. Prolonged survival a t 36 h ( p = 0.009) was seen in animals receiving both ampicillin and low dose polymyxin compared to either ampicillin dose alone. Ampicillin significantly reduced the number of bacteria in blood of survivors ( p < 0.023 a t 30 min) compared to untreated animals but increased the activity of free endotoxin a t 30 min compared to controls ( p = 0.006). I n vitro endotoxin release from H. influenzae type b increased 5-fold after addition of 100 pg/rnl of ampicilin, whereas a six-fold reduction in endotoxin activity was measured after the addition of 7 pg/ml of polymyxin B. Subinhibitory doses of polymyxin B modulate the ethal effects of overwhelming H. influenzae type b infection in infant rats and might be beneficial a s adjunct treatment in gram-negative septicemia. (Pediatr Res 20: 237-241,1986 philic polysaccharide, a core, and a hydrophobic lipid component, the lipid A. The polysaccharide determines the immunological specificity of the given LPS whereas lipid A, also endowed with antigenic properties, is responsible for the biologic activities of LPS (1). The role of endotoxin in the pathogenesis of shock associated with gram-negative bacteria in humans is not clear; however, the fact that antibodies directed against the core region of LPS significantly decrease mortality in humans with gram-negative infections supports the probability of some contribution of endotoxin to the physiologic effects seen in septic shock (2).Shenep et al. (3,4) have demonstrated the release of endotoxin from Escherichia coli by antimicrobial agents in a rabbit model. Some children with Hib meningitis, who initially have normal peripheral perfusion, develop septic shock within 1 to 2 h after receiving their first dose of antibiotics. The LPS of Hib is similar in biological action to LPS from other gram-negative bacteria (5). Further indirect evidence that LPS contributes to the virulence of Hib includes the observation that antibodies against the related E. coli J5 LPS core antigen protects six to 8-wk-old mice against death due to Hib infection (6).Polymyxin B is a cationic, cyclic polypeptide antibiotic that specifically binds to the lipid A-core region of LPS, disrupting the outer membrane. This action neutralizes endotoxin effects in vitro and in vivo (7)(8)(9)(10)(11)(12)(13)(14)(15)(16). However, the neutralizing effect was only seen if polymyxin B was administered in advance or in conjunction with artific...

Clinical relevance of antibiotic-induced endotoxin release

“…In general, such studies showed a decrease in bacterial counts concurrent with an increase in endotoxin levels in antibiotic-treated animals or a larger increase in endotoxin levels in treated than in untreated animals. This was demonstrated in experimental E. coli sepsis in rabbits (65,69) and pigs (63), H. influenzae sepsis in rats (79), meningococcal sepsis in mice (20), and E. coli (35,73) Parallel with a higher increase in endotoxin levels in antibiotic-treated animals, some studies reported higher TNF levels (35,54,57), but a correlation between a rise in endotoxin level and interleukin 6 levels could not be demonstrated (23). Although corticosteroids appear to have no influence on the process of antibiotic-induced endotoxin release (31,57), the TNF response of the host could be attenuated by dexamethasone (54,57).…”

“…At subinhibitory levels, it reduced mortality in rats with H. influenzae type b infection (79), it reduced the effects of antibiotic-induced endotoxin release in a meningitis model (73), and it attenuated acidosis and hemodynamic changes in rabbits with experimental E. coli sepsis (30).…”

“…During treatment of Haemophilus influenzae type b meningitis, peptidoglycans released from the bacterial cell wall into the cerebrospinal fluid (CSF) also contribute to meningeal inflammation (12). Jarisch-Herxheimer reactions that occur during treatment of secondary syphilis were proven not to be mediated by endotoxin (66 (44,79), most of the bacteria were killed within 2 h, whereas endotoxin levels still increased after 6 h. In two studies (52,78), the increase in endotoxin concentrations leveled off after 1 to 2 h, whereas bacterial counts still decreased. In two other studies (9,25), the time course of bacterial killing and endotoxin release occurred in parallel.…”

Adjunctive Treatment with Low-Dose Polymyxin B Improves Operant Response Rates and Latency in Infant Rats with Haemophilus influenzae Type b Meningitis

“…Numerous (18). The literature seems more notable for the paucity of reports describing failure with polymyxin B under various experimental conditions than the many descriptions of its mitigating effects.…”

“…Another way is to aim for the chemical neutralization of endotoxin. An agent that is approved for use in humans, polymyxin B, was successful in this respect by decreasing mortality in infant rats when given concomitantly at low doses with antimicrobial treatment in overwhelming infection due to H. inj7uenzae type b (18). The next study was designed to address the question of whether such an approach could improve the neurofunctional outcome.…”