Objective: To evaluate the potential of biotherapeutic agents (microorganisms with therapeutic properties) for the prevention and/or treatment of selected intestinal and vaginal infections.
Data Sources: The MEDLINE database was searched for all relevant articles published between 1966 and September 1995. Search terms used were biotherapeutic agent, probiotic, Lactobacillus, Saccharomyces, Bifidobacterium, Candida, gastrointestinal-system, vaginitis, vaginosis-bacterial, and related terms. The bibliographies of obtained articles were also reviewed.
Study Selection and Data Extraction.All placebo-controlled human studies on biotherapeutic agents were reviewed. English-language open trials, case series and reports, and animal studies were reviewed only if they were especially relevant to providing information on the potential efficacy, adverse effects, or mechanisms of action of these agents.
Data Synthesis: Placebo-controlled studies have shown that biotherapeutic agents have been used successfully to prevent antibiotic-associated diarrhea (Lactobacillus casei GG, Bifidobacterium longum, B longum with L acidophilus, and Saccharomyces boulardii), to prevent acute infantile diarrhea (Bifidobacterium bifidum with Streptococcus thermophilus), to treat recurrent Clostridium difficile disease (S boulardii), and to treat various other diarrheal illnesses (Enterococcus faecium SF68, L casei GG, and S boulardii). There is also limited evidence for Lactobacillus acidophilus in the prevention of candidal vaginitis. Few adverse effects have been reported. However, many of the studies tested only small numbers of patients or volunteers.
Conclusions: There is now evidence that administration of selected microorganisms is beneficial in the prevention and treatment of certain intestinal and, possibly, treatment of vaginal infections. In an effort to decrease the reliance on antimicrobials, the time has come to carefully explore the therapeutic applications of biotherapeutic agents. (JAMA. 1996;275:870-876)
Challenges in the control and treatment of infectious diseases include the development of antibiotic resistance, increased frequency of opportunistic infections in immunocompromised patients, and emergence of new types of pathogens. These trends result in an increase in antibiotic exposure in the population, which in turn selects for emergence of resistant strains of pathogens. There is a need to develop new strategies for treatment and prevention of infectious diseases.
Crude mixtures of microorganisms (eg, fermented milk products or poultices of moldy bread) have been used since antiquity to treat infections. With the advent of antimicrobials, the use of this approach diminished. Research conducted mainly in Europe and more recently in the United States provides evidence for the therapeutic use of whole microorganisms with antagonistic activities against troublesome pathogens. The therapeutic uses of these microorganisms are primarily confined to infections involving nonsterile mucosal surfaces such as the gut and the vagina. Ideally, these microorganisms would be innocuous, act against pathogens by multiple mechanisms (thus minimizing the development of resistance), and marshal the host defenses to destroy the invading pathogen. An additional desirable property would be an immediate onset of action (in contrast to a vaccine that may take several weeks to stimulate antibody production). Such microbes may be given alone or in conjunction with an antimicrobial (to which it is not sensitive). The terms "probiotic" [1] and "biotherapeutic agent" [2] have been used in the literature to describe microorganisms that have antagonistic activity toward pathogens in vivo. This report uses the term "biotherapeutic agent," a more appropriate term to denote microorganisms having specific therapeutic properties, and focuses largely on a review of recent controlled studies for treatment or prevention of infectious diseases in humans.
PREVENTION OF DIARRHEA Antibiotic-associated diarrhea (AAD) is the most common adverse effect of antimicrobial therapy, occurring in 3.2% to 29% of hospitalized patients [3-5]. It has been associated with an increased number of days of hospitalization, higher medical costs, a fivefold increased risk for other nosocomial infections, and a threefold increased mortality [4-6]. At least one third of AAD is associated with Clostridium difficile [3,7], but the etiology of the remainder is uncertain. The risk for C difficile-associated diarrhea is related to the perturbation of the normal microbial ecology of the gastrointestinal tract by antimicrobial treatment.
Several clinical trials have used biotherapeutic agents, including the nonpathogenic yeast Saccharomyces boulardii and several genera of bacteria including Lactobacillus and Bifidobacterium for the prophylaxis of AAD. A summary of placebo-controlled human studies evaluating biotherapeutic agents is shown in the ( Table 1). A large, double-blind, placebo-controlled trial of S boulardii (Laboratoires Biocodex, Montrouge, France) in 388 ambulatory adults receiving either tetracycline or beta-lactam antibiotics for at least 5 days showed significantly fewer patients with AAD (9/199, 4.5%) compared with patients receiving placebo (33/189, 17.5%; P<.001) [8]. The effectiveness of S boulardii in AAD was more recently tested in two double-blind, placebo-controlled trials in the United States [9,10]. Hospitalized patients (n=180) receiving new antibiotic prescriptions were assigned either to S boulardii (500 mg of lyophilized yeast twice a day, 1x1010 CFU (colony-forming units)) or placebo given concurrently with the antibiotic and continued for 2 weeks after discontinuation of the antibiotic [9]. Significantly fewer of the patients receiving S boulardii developed AAD (11/116, 9.5%) compared with those receiving placebo (14/64, 22%; P=.04). The calculated efficacy of S boulardii in preventing AAD was 56.7%. This finding was confirmed in another study of 193 patients receiving at least one broad-spectrum beta-lactam antibiotic. Of the 97 patients receiving S boulardii, only 7.2% developed AAD compared with 14.6% of the 96 patients receiving placebo, with a calculated efficacy of 51% [10]. No significant adverse reactions were noted.
Bifidobacterium longum reduced AAD in healthy volunteers receiving erythromycin. The study was a crossover design with 10 volunteers receiving B longum yogurt or placebo yogurts three times daily for 3 days [16]. There was a significant reduction (P<.005) in stool weight (208 g/d vs 145 g/d, respectively) and stool frequency (mean 1.9 bowel movements per day vs 1.2 per day, respectively) (P<.03) when the volunteers were in the B longum phase compared with the placebo yogurt phase. Abdominal discomfort was experienced by six of 10 in the placebo phase vs 1 of 10 in the treatment phase (P=.03). A weakness of this trial was that the treatment time was only 3 days. Orrhage et al reported a significant decrease (P<.05) in gastrointestinal discomfort in 10 volunteers receiving clindamycin plus a fermented milk product containing B longum and L acidophilus compared with 10 volunteers receiving a fermented milk product without these strains [17]. A commercial lactic acid-producing preparation of Enterococcus faecium SF68 or placebo was given prophylactically for 7 days to 45 patients receiving antibiotics in a multicentered, double-blind trial [18]. Diarrhea was observed in 27.2% of the placebo group compared with 8.7% of the treated group (P=.11).
Saavedra et al reported the results of a double-blind, placebo-controlled trial of an infant formula supplemented with Bifidobacterium bifidum and Streptococcus thermophilus compared with the same formula unsupplemented, for the prevention of acute diarrhea in a hospital setting [19]. Eight of 26 infants fed control formula developed diarrhea compared with two of 29 receiving the supplemented formula (P<.04). Fewer treated infants shed rotavirus during the study.
PREVENTION OF TRAVELER'S DIARRHEA Lactobacillus casei GG was evaluated in the prevention of traveler's diarrhea in 756 Finnish tourists traveling to Turkey [20]. Overall, the rates of traveler's diarrhea were similar in patients receiving L casei GG (41.0%) and placebo (46.5%) (P=.07). When only one of the two destination cities was considered, the rates were significantly lower in the Lactobacillus group (23.9%) compared with the placebo group (39.5%) (P=.04). Unfortunately, the etiologies of the diarrhea were not characterized, and the subset analysis may have given an erroneous result; thus, it is difficult to determine why Lactobacillus was effective in one city and not the other. A double-blind, placebo-controlled trial using S boulardii was conducted with 3000 Austrian travelers [21]. In travelers compliant with the protocol (n=1016), the rates of diarrhea were 39.1% in the placebo group and 28.7% in the group receiving 1 g/d of the active yeast (P<.005). At 250 mg/d the yeast was less effective (34.4% diarrhea; P<.02). Like the above-mentioned Lactobacillus study, there were regional effects and no information was obtained on the pathogens involved. These biotherapeutic agents may have some application in preventing traveler's diarrhea, but focused studies with pathogen identification are needed to pinpoint the means by which they are efficacious. Even though therapeutic effects were rather small in the two studies, further investigation is warranted, because an effective biotherapeutic agent would have the potential of reducing exposure to fluoroquinolones and other antimicrobials frequently used for traveler's diarrhea. In addition, these therapies should be compared with bismuth products, which have the advantages of being inexpensive and readily available.
TREATMENT OF C DIFFICILE DIARRHEA Clostridium difficile is a classic example of an opportunistic intestinal pathogen that proliferates following antibiotic disruption of normal colonic flora. Infection with C difficile may result in diarrhea, colitis, pseudomembrane formation, toxic megacolon, and, in severe cases, death. This infection is usually responsive to treatment with vancomycin or metronidazole, although recurrent C difficile disease can occur [22]. In an effort to reestablish normal colonic flora in patients with recurrent C difficile infections, fecal infusions from normal hosts have been moderately successful [23-25]. This approach, however, suffers from aesthetic problems and has risks of transmission of undetected pathogens, including the human immunodeficiency virus (HIV).
Lactobacillus casei GG has been evaluated in two small open trials in recurrent C difficile colitis. In a trial of five adult patients, four responded (no further relapses) to a treatment with 1010 CFU/d of L casei GG for 7 to 10 days [26]. In a second trial, four children with C difficile colitis having a history of multiple relapses were treated with this Lactobacillus for 2 weeks [27]. Although no other antimicrobials were given, all of the children were asymptomatic at the end of treatment. Two patients relapsed again but were successfully retreated.
In another open trial, 13 patients with recurrent C difficile infections were treated with vancomycin (1 g/d for 10 days) and S boulardii (1 g/d for 28 days) of whom 11 had no further disease [28]. In a subsequent larger, placebo-controlled study, 124 adults with active C difficile disease were treated with standard antibiotic regimens (vancomycin or metronidazole) with an adjunct of either S boulardii (1 g/d for 28 days, n=57) or placebo (n=67). Significantly fewer (26%) of the S boulardii group experienced a recurrence of disease within the 2 months of follow-up compared with 45% of the placebo group (P=.05) [29]. The only adverse reactions associated with the yeast were increased thirst and mild constipation.
With respect to prevention of C difficile disease, the difficult question to address is whether routine use of biotherapeutic agents with antimicrobials is justified. Clinical experience and prior studies have identified some patients who are at increased risk for C difficile infections, ie, patients with prolonged hospitalizations, especially if they have had recent gastrointestinal surgery and/or procedures, and elderly women receiving multiple or broad-spectrum antibiotics for prolonged periods [30]. However, there is a significant risk of diarrhea and even pseudomembranous colitis in otherwise healthy outpatients. In our ongoing studies of recurrent C difficile disease, the majority of those affected were outpatients receiving antibiotics for common, non-life-threatening infections such as sinusitis or upper respiratory tract infections (Surawicz et al, unpublished findings, 1995). While AAD is common and often resolves upon discontinuation of the antibiotic, there is no way at present to identify the much smaller number of individuals who will go on to develop C difficile colitis or pseudomembranous colitis. At present, evidence is lacking to show that any biotherapeutic agent will prevent C difficile disease. The routine use of a biotherapeutic agent as an adjunct to antimicrobial therapy to prevent C difficile colitis cannot be recommended at present; however, further studies are warranted.
TREATMENT OF OTHER DIARRHEAS Thirty-five patients infected with HIV who had chronic diarrhea were enrolled in a double-blind, placebo-controlled trial using S boulardii (1.5 g, twice daily for 1 week) or placebo. Ten (56%) of the 18 given S boulardii had resolution of the diarrhea compared with significantly fewer (1/17, 6%) of the patients on placebo (P<.001) [31]. In a study by Elmer et al, 11 patients with the acquired immunodeficiency syndrome (AIDS) who had chronic diarrhea and no identifiable pathogen showed no significant change during a 7-day, double-blind, placebo-controlled phase of a study using S boulardii [32]. However, seven of 11 responded during weeks 2 to 4 when all patients were given the active yeast. Seven patients continued on S boulardii for 1 to 15 months with either resolution of diarrhea or clinical improvement in diarrhea. Doses greater than 1.5 g/d were needed for an antidiarrheal effect. Given the seriousness of AIDS-related diarrhea and the paucity of effective treatment modalities, the efficacy of S boulardii and other biotherapeutic agents is worthy of careful evaluation in larger controlled trials in AIDS patients. One may question the rationale of administering biotherapeutic agents for AIDS enteropathy and other small intestinal infections, since there are no true stable microflora for most of the small intestine and the residence time of a biotherapeutic agent at this site is probably brief. While AIDS enteropathy, by definition, is limited to the small intestine [33], many opportunistic pathogens in the small intestine such as cryptosporidia and cytomegalovirus may also affect the colon. Furthermore, small intestine bacterial overgrowth causes diarrhea in stasis syndromes such as pseudo-obstruction and has been postulated to contribute to AIDS diarrhea [34] as well as diabetic diarrhea. In such situations, investigations are needed to determine whether biotherapy, with attendant competition for nutrients or other mechanisms, may be beneficial.
Saccharomyces boulardii (0.5 g/d for 5 days) with standard oral rehydration was compared with rehydration alone in 38 children with acute diarrhea [35]. Comparison between treated and control groups on days 1 and 4 showed a significant (P<.05) decrease in stool weight and number and an increase in carmine red transit time with yeast treatment. A large (n=130), placebo-controlled, double-blind study of children with acute diarrhea showed a significant effect of S boulardii in reducing the number of stools and increasing the number of cures (P<.05) [36]. Hochter et al reported a significant therapeutic effect of S boulardii compared with placebo in a multicentered, double-blind study of 92 adult outpatients with acute diarrhea (P=.04) [37]
Two trials have used E faecium SF68 in similar doses to treat acute diarrhea with discordant results. Wunderlich et al [18] evaluated E faecium for treatment of 78 patients with acute diarrhea. Patients given E faecium reported less diarrhea (0.6%) on day 7 than did patients on placebo (8.7%) (P<.01). In the other study, 183 adults in Bangladesh with diarrhea (114 with Vibrio cholerae, 41 with enterotoxigenic Escherichia coli, and 28 with unknown etiologies) were treated with the E faecium SF68 or placebo (nonviable E faecium). There was no difference in the resolution of diarrhea between the two groups [38]. Because of the brevity of this study (3 days), it may be inappropriate to conclude that E faecium is ineffective in the severe diarrhea caused by V cholerae and enterotoxigenic E coli.
Lactobacillus casei GG (as fermented milk or lyophilized powder) was tested in a placebo-controlled trial of 71 children with acute diarrhea (82% with rotaviral infections) [39]. After oral rehydration, patients were randomly assigned to receive 5 days of either L casei GG product or placebo (pasteurized yogurt). The duration of diarrhea was significantly shorter in both the Lactobacillus groups (1.4+or-0.8 days) than in the placebo group (2.4+or-1.1 days) (P<.001). Two studies of L casei GG in premature infants, however, showed no reduction in intestinal pathogens or clinical benefit in this population [40,41]. In another study of 78 children with persistent diarrhea, treatment with yogurt containing L bulgaricus and S thermophilus was compared with a standard milk diet [42]. Fewer of the yogurt-fed children failed to respond to treatment (15%) compared with the milk formula fed children (45%). Taken as a whole, there appears to be a preponderance of evidence that biotherapeutic agents may have significant efficacy in treating acute diarrhea. Further large, placebo-controlled trials are required to compare agents and to define etiologies that would most benefit from this treatment approach.
CANDIDA VAGINITIS While most acute vaginal infections are easily treated with nontoxic and inexpensive antimicrobial agents, chronic infections and frequent recurrences are a common manifestation of candidiasis, the most common type of vaginal infection. Similar to the situation with the gastrointestinal tract, antibiotic perturbation of the vaginal flora places the patient at risk for pathogen overgrowth. On theoretical grounds, it would appear that biotherapeutic agents might be useful in the treatment and prevention of some vaginal infections.
Lactobacilli are major constituents of the normal vaginal flora and are thought to resist pathogen colonization through the production of bactericins, lactic acid, and hydrogen peroxide [43-45]. There has long been empirical, self-prescribed use by the public of yogurts and other nonprescription products containing lactobacilli to control vaginitis, but only one recent controlled study has been performed. A yogurt containing more than 108/mL of a hydrogen peroxide-producing strain of L acidophilus was tested by Hilton et al in a crossover design study (6 months with no yogurt, 6 months of 240 mL (8 oz) of yogurt taken orally daily) [46]. Of the 33 women with recurrent candidal vaginitis enrolled, only 13 completed the study on account of problems with attrition, protocol violations, and refusal to enter a no-yogurt arm after completion of the yogurt arm of the study. The mean+or-SD number of infections per 6 months was 2.54+or-1.66 in the control arm and significantly fewer (0.38+or-0.51) during the Lactobacillus arm (P=.001). Also, the mean+or-SD number of colonizations with Candida per 6 months was decreased from 3.23+or-2.17 to 0.84+or-0.90 during the yogurt arm (P=.001). Of particular interest for the application of oral biotherapeutic agents was the observation that there was a significant (P<.001) relationship between the presence of L acidophilus in the rectum and its presence in the vagina. Recently Hilton et al [47] reported subjective improvement with 7-day use of L casei GG vaginal suppositories in an open trial of 28 women with recurrent candidal vaginitis.
McGroarty [48] and Reid et al [44] have reviewed many of the earlier reports showing conflicting results in the evaluation of yogurt and Lactobacillus preparations for vaginitis. Of direct bearing on the interpretation of the literature and for future studies is the finding of Hughes and Hillier [49] showing a serious lack of quality control for the commercial nonprescription Lactobacillus products. In order to define the efficacy for vaginitis, the investigator will need to use a standardized product containing high viable counts of hydrogen peroxide-producing strains of lactobacilli. More work is needed to define the efficacy and the best route of administration for lactobacilli and other biotherapeutic agents for vaginitis.
URINARY TRACT INFECTIONS Biotherapeutic agents may also have an application in urinary tract infections. Reid et al studied the use of Lactobacillus vaginal suppositories on urinary tract infection recurrences following a 3-day antimicrobial regimen [50]. The recurrence rate in the control suppository group was eight per 17 episodes compared with three per 14 episodes in the Lactobacillus-treated group (P=.27). It was not clear from this small study whether the lactobacilli colonized the urinary tract or whether the role of the lactobacilli was to create an environment in the vaginal tract that would minimize transfer of pathogens to the urinary tract. Long-term prophylaxis for recurrent urinary tract infections with antimicrobials carries risks of development of resistance and perturbations of the normal protective microflora; however, it should be noted that long-term use of trimethoprim-sulfamethoxazole for more than 5 years demonstrates continuing efficacy, minimal resistance and inconvenience, and no clear impact on the gastrointestinal flora. Nitrofurantoin may also be considered as a treatment possibility. Comparisons of efficacy between trimethoprim-sulfamethoxazole, nitrofurantoin, and biotherapeutic agents would be desirable. Further investigation of defined biotherapeutic agents for the prevention of recurrent urinary tract infections seems worthwhile.
MECHANISMS OF ACTION OF BIOTHERAPEUTIC AGENTS The development of diarrhea, urinary tract infections, or vaginitis may occur when the normal flora are disrupted by antibiotics, medications, or medical procedures. Colonization resistance is the ability of normal flora to protect against the unwanted establishment of pathogens. Colonization resistance is attributable to a complex interaction of many of the individual bacteria that comprise the mucosal microflora. Attempts to identify and use a single microorganism or a defined mixture of microorganisms that would have the broad specificity of the normal microflora to resist pathogen invasion have not been successful. An important goal of therapy with biotherapeutic agents is to stop proliferation of the pathogen until such time that the normal microflora can be reestablished. The ability to "buy time" to reestablish colonization resistance is one probable important mechanism of successful therapy with biotherapeutic agents.
The production of antimicrobial substances by various biotherapeutic agents has been demonstrated. Lactobacillus casei GG has been shown to produce a microcin inhibitory in vitro toward a broad spectrum of gram-positive and gram-negative pathogens [51], and hydrogen peroxide, which is bactericidal. Other inhibitory metabolic products of lactobacilli have been reported [52,53]. While the presence of these antimicrobial metabolites has been demonstrated in vitro, it is unclear whether they are produced or have activity in vivo. However, Ramare et al showed the presence of an antibacterial substance in the feces of gnotobiotic rats monoassociated with a human Peptostreptococcus strain [54]. This compound appeared to be produced through the concerted action of the host trypsin and a compound produced in vivo by the Peptostreptococcus.
Competition for nutrients has been proposed as another mechanism for biotherapeutic agents. Clostridium difficile is dependent on monosaccharides for growth, and the addition of homogenates of mouse cecal contents in continuous cultures inhibited C difficile growth unless monomers were added [55]. Dietary effects on the progression of C difficile disease in animal models have been reported [56,57], but the extent of microflora involvement in this effect is not clear.
Competitive inhibition for bacterial adhesion sites is another possible mechanism of action for biotherapeutic agents. A Lactobacillus strain was shown to compete with enteropathogenic E coli in pig ileum and also interfered with bacterial attachment to the mucous layer of ileal conduits [58]. Lactobacillus acidophilus inhibits the adhesion of several enteric pathogens to human intestinal cells in culture [59,60]. When pathogen attachment preceded L acidophilus treatment, no interference occurred indicating that steric hindrance is important in the inhibition of adhesion. Exposure of Entamoeba histolytica trophozoites to S boulardii, its membranes, or to yeast culture supernatants decreased the numbers of trophozoites able to attach to erythrocytes in vitro [61]. Earlier experiments [62] showed that treatment with S boulardii decreased mortality in young rats infected with E histolytica.
Another related mechanism of biotherapeutic agents is the ability to enzymatically modify a toxin receptor. Pothoulakis et al described an enzyme from S boulardii that degraded the C difficile toxin A receptor in rabbit ileum [63]. Exposure either to S boulardii or to S boulardii-conditioned culture medium decreased the pathological effects of toxin A in isolated rabbit ileum. Vidon et al reported that S boulardii inhibited cholera-induced secretion in rabbit jejunum [64]. Using cultured intestinal epithelial cells, Czerucka and coworkers probed the mechanism of this effect. Viable cells of S boulardii and S boulardii-conditioned medium but not cells of a related Saccharomyces species, Saccharomyces pombe, reduced cholera toxin-induced cAMP levels [65]. The S boulardii-conditioned medium also decreased cAMP induced by E coli heat-labile toxin and by forskolin (a diterpene from Coleus forskolii). The yeast activity was associated with a 120-kd heat- and trypsin-labile protein. Interestingly, the effect of pertussis toxin was not neutralized by S boulardii nor was the cholera toxin modified by the yeast treatment. Studies using S boulardii indicate that interactions with host cell receptors may be important in reducing the pathological effects of infections.
Studies by Buts and coworkers point to the involvement of the host as a factor in the therapeutic activity of S boulardii [66-68]. Yeast treatment of rats and human volunteers resulted in a trophic effect with respect to increasing mucosal disaccharidase activities [66]. Increased secretion of rat secretory IgA and the secretory component of immunoglobulins was also observed [67]. Further work revealed that polyamines released by S boulardii in vivo are most likely responsible for the trophic effects on the small intestine of weanling rats [68]
Ducluzeau and Bensaada demonstrated that oral S boulardii treatment of gnotobiotic mice decreased proliferation of systematically administered Candida albicans, presumably by some type of immune stimulation [69]. A study by Caetano et al has further defined the systemic immunological changes observed upon oral administration of S boulardii [70]. Ninety-six healthy volunteers received an oral yeast dose of 1 g/d for 7 days. At day 8, a pattern of significant cellular and humoral changes was observed that led these investigators to conclude that S boulardii activates both the complement and reticuloendothelial systems. Immune stimulation by L casei [71], L casei GG [72], and E coli [73] has also been reported. Thus, systemic effects upon oral administration of biotherapeutic agents may also be involved in their activity.
While the ability to survive the digestive processes in the gastrointestinal environment is thought to relate to efficacy, proliferation and persistence in the gut are apparently not of paramount importance for biotherapeutic agents, provided continuous daily administration is performed. Lactobacillus casei GG is the only biotherapeutic agent shown to persist in the intestinal tract following cessation of dosing. At day 7 after discontinuing administration, six of 18 subjects still harbored L casei GG, albeit with a 100-fold decrease from steady state levels [74]. However, other biotherapeutic agents with demonstrated efficacies (eg, L acidophilus [75], S boulardii [76], or a Bifidobacterium species [77]) survive intestinal passage as evidenced by high fecal levels but disappear within a few days after the last dose. If consumed on a daily basis, the beneficial effect of the transient biotherapeutic agents can be maintained.
While considerable work remains in order to clarify the mechanisms by which biotherapeutic agents are of benefit in infectious diseases, work to date indicates that different mechanisms of action may exist depending on which pathogen is involved. Furthermore, the host systemic immune response to biotherapeutic agents may also be important to their therapeutic activities. Thus, the biotherapeutic agent may well be viewed as a vehicle to deliver pathogen inhibitory activities and/or host immune stimulant activities to the intestinal tract.
ADVANTAGES AND DISADVANTAGES OF BIOTHERAPEUTIC AGENTS Biotherapeutic agents most likely act by multiple mechanisms; thus, the risk of pathogens' developing resistance to biotherapeutic agents by point mutations is diminished. Available biotherapeutic agents are not usually absorbed, and those having the ability to survive transit through the upper gastrointestinal tract offer a means to deliver metabolic activity to the colon. For the future, administration of the identified active substance(s) produced by the biotherapeutic agents may be desirable, such as has been shown by the inhibition of enterotoxigenic E coli attachment upon administration of an enteric-coated protease preparation [78]. Furthermore, genetic engineering offers the possibility of using microbes to deliver specific actions or products to the colon or other mucosal surfaces. No serious adverse effects have been reported with the biotherapeutic agents tested to date; however, there is limited information on the use of biotherapeutic agents in immunocompromised hosts. The technology needed to produce pharmaceutical-grade biotherapeutic agents would not appear to be complex, and it seems possible that biotherapeutic agents could be made available at a reasonable cost in the future.
The disadvantages of biotherapeutic agents at present relate to the lack of knowledge and experience with these products. There have been few large, controlled, double-blind clinical trials, and the number of agents evaluated has been limited. An obvious concern in the use of biotherapeutic agents is administration to severely immunocompromised or debilitated patients such that invasion into the systemic compartment might occur. A case of fungemia in a 1-year-old girl treated for prolonged diarrhea with S boulardii has been recently reported [79]. Lactobacilli in the form of fermented dairy products appear innocuous; however, as special strains are selected for their therapeutic rather than fermentative properties, the usual safety concerns arise. In order for a bacterial biotherapeutic agent to be effective when given concurrently with antibiotics, the agent should be resistant to the antibiotic; this property raises questions as to whether resistance genes can be transferred and what impact the transfer would have on subsequent antimicrobial therapy. McConnell et al [80] have demonstrated transfer of plasmid pAMbeta1 (macrolide-lincosamide-streptogramin type B resistance) from Lactobacillus reuteri to E faecium and from E faecium to Enterococcus faecalis in the mouse intestinal tract. A special strain of E faecium SF68 is being used as a biotherapeutic agent [18]. Enterococcus faecium infections are especially dangerous because many strains carry plasmids coding for resistance to multiple antibiotics including vancomycin. The safety and long-term effects on antibiotic resistance of treatment with E faecium SF68 need careful assessment before widespread use of this biotherapeutic agent is undertaken.
Definition of the pharmacodynamic profiles of biotherapeutic agents has received little attention to date. Studies to optimize dose regimens and delivery vehicles are needed. This lack of scientific data on biotherapeutic agents has led to skepticism as to their utility in human disease.
CONCLUSIONS Biotherapeutic agents may offer an alternative to conventional antimicrobials to which many pathogenic microorganisms eventually develop resistance. Based on experience with existing biotherapeutic agents, their advantages may include low risk, presumed low cost, and likely existence of multiple mechanisms of action for a single agent. Production and formulation of biotherapeutic agents should be relatively simple and inexpensive.
For human intestinal diseases, biotherapeutic agents have been used successfully with few adverse effects to prevent antibiotic-associated diarrhea (L casei GG [15], B longum [16], B longum with L acidophilus [17], and S boulardii [9,10]), to prevent acute infantile diarrhea (B bifidum with S thermophilus [19]), to treat recurrent C difficile disease (Lactobacillus GG [26,27] and S boulardii [28,29]), and to treat various diarrheal illnesses (E faecium SF68 [18], L casei GG [39], and S boulardii [31,35-37]). There is also limited evidence for the application of lactobacilli [46] in the prevention of candidal vaginitis and L casei GG and S boulardii in the prevention of traveler's diarrhea [20,21]
Although promising, many of the studies were uncontrolled and tested only small numbers of patients or volunteers. Larger controlled trials are needed to prove efficacy for some of these agents. Beyond the initial clinical trials of effectiveness against various diseases, biotherapeutic agents need to be characterized with the same toxicity, dose-ranging, and pharmacokinetic studies expected for standard chemical investigational drugs. The published studies have rarely presented data on dose-dependent efficacy, absorption, distribution, metabolism, excretion, and duration of effect. There are some pharmacokinetic data from humans on S boulardii [76,81], Bifidobacterium [77], and L casei GG [74], but further studies are needed to define how they should be best administered.
Historically, most studies on biotherapeutic agents have been carried out in Europe. It may well be time for researchers elsewhere to recognize the potential of these agents and to evaluate their efficacy in the prevention and treatment of infectious diseases. For example, one area that could benefit from the application of biotherapeutic agents is the treatment of chronic diarrhea associated with AIDS. In many cases there is no demonstrated cause or there may be no therapy to eradicate identified pathogens such as cryptosporidia. Saccharomyces boulardii may well be effective and should be studied along with other agents to alleviate diarrhea and the associated malnutrition and weight loss. Other fertile areas for future study include defining the mechanisms of action of various biotherapeutic agents with the possibility of applying genetic engineering to enhance activities. Finally, in vivo metabolic and pharmacokinetic studies are needed to improve drug formulation, dosing, and delivery.
The use of biotherapeutic agents to treat infectious diseases may well become an important area for further drug development and is worthy of significant research attention.
Dr McFarland is employed part-time by Biocodex, Inc, which manufactures one of the biotherapeutic agents discussed in this article, but she has no stock or equity in the company. Drs Elmer and Surawicz are paid consultants for Biocodex, but hold no stock or equity in the company.
Reprint requests to Department of Medicinal Chemistry, PO Box 357610, University of Washington, Seattle, WA 98195 (Dr Elmer).
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Bifidobacterium; Candida; Clostridium difficile; Diarrhea; Lactobacillus; Preventive Medicine; Saccharomyces; Urinary Tract Infections; Vaginitis; Vaginosis, Bacterial
