The sulfonamides are one of the oldest groups of antimicrobial compounds still in use today. Sulfanilamide, an amide of sulfanilic acid, was the first sulfonamide used clinically. It was derived from the azo dye Prontosil. Other sulfonamides also share the same structure and the “sulfonamide” structure is prevalent among other drug classes, including nonsteroidal anti-inflammatory drugs (NSAIDs), anticonvulsants, and diuretics. Sulfonamide antimicrobials have been in clinical use for 50 years, but resistance is common when these drugs are used alone (without addition of trimethoprim or ormetoprim).
Clinical use of sulfonamides in dogs, cats, horses, and some exotic and zoo animals usually rely on the addition of trimethoprim (trimethoprim–sulfonamide) or ormetoprim (e.g., ormetoprim–sulfamethazine) to broaden the spectrum and increase antibacterial activity against bacteria that are resistant to either drug used alone. Technically, trimethoprim and ormetoprim are chemically called diaminopyrimidines, but they will be referred to by their respective names in this chapter. In companion animals, trimethoprim–sulfonamide combinations have all but replaced single or combination sulfonamide (triple-sulfas) treatment regimens. Sulfonamide administration is restricted in food animals, particularly dairy cattle, because of a concern for drug residues.
Pharmacology of SulfonamidesAll sulfonamides are derivatives of sulfanilamide (structurally similar to para-aminobenzoic acid), which was, in the 1940s, the first sulfonamide discovered to have antimicrobial activity. Note that in some countries and certain formularies outside the United States, different spellings have been used for sulfonamides (e.g., sulphamethoxazole for sulfamethoxazole; sulphadiazine for sulfadiazine; sulphadimethoxine for sulfadimethoxine, and so forth).
Many structural derivatives of sulfanilamide with differing pharmacokinetic and antimicrobial spectrums have been used in veterinary medicine to treat microbial infections of the respiratory, urinary, gastrointestinal, and central nervous systems.
Susceptible organisms include many bacteria, coccidia, chlamydia, and protozoal organisms, including Toxoplasma spp.
Sulfonamides are white crystalline powders that are weak organic acids, with solubility in water that varies among the specific drugs (ranging from slightly soluble to practically insoluble), and have a wide range of pKa values. The pKa values of these compounds and their ionization are important because – among other properties – the antibacterial activity, solubility, and protein binding have been associated with the pKa value (Mengelers et al., 1997). Drugs with high pKa are less soluble and exhibit lower protein binding; drugs with low pKa tend to have higher protein binding. The sulfonamides all share a similar structure, which contains a –SO2 group linked to a benzene ring, and a para NH2– group on N-4. An attached pyrimidine ring may contain zero, one, or two methyl groups (sulfamethazine, sulfamerazine, and sulfadiazine, respectively), which may undergo hydroxylation during metabolism. The other major site of metabolism is acetylation of the para-NH2, which can vary among species (for example, dogs do not acetylate, which is discussed in Section Metabolism). Acetylated forms of the drug tend to be less soluble.
The pKa is the dissociation rate constant. For some drugs, more than one pKa value is listed because of variation among sources. For pKa values, all sulfonamides are weak acids; trimethoprim and ormetoprim are weak bases. Log P is the logarithm of the partition coefficient between an organic solvent (oil) and water. The higher the Log P, the more lipophilic is the drug. Some values are from Mengelers et al. (1997) and van Duijkeren et al. (1994a).
The sulfonamides exhibit large variation in the extent to which they bind to plasma proteins. In general, the plasma protein binding is higher than other antimicrobials (>70% in many animals), and ranges from 90% (sulfadimethoxine in some species) to as low as 50% (sulfamethazine in some species). In horses, the protein binding of trimethoprim was 20–30% and for sulfadiazine was 18–30% (Winther et al., 2011). Because they are weak acids, sulfonamides are more soluble in alkaline than in neutral or acidic pHs; water solubility is enhanced when the sulfonamides are formulated as sodium salts or when in solution in more alkaline environments. Some sulfonamide solutions have pHs between 9 and 10, prohibiting extravascular use. Because solubility is decreased in acidic pH, they may become particularly insoluble and crystallize in renal tubules when urine pH is low, especially when high doses are administered, or animals are dehydrated or acidemic. To minimize crystalluria, yet allow administration of high doses, they have been formulated in combination with other sulfonamides. Each sulfonamide in a mixture of sulfonamides exhibits its own solubility in solution (law of independent solubility); that is, sulfonamides do not significantly affect the solubility of each other, but the antimicrobial effect is additive; thus, the use of “triple-sulfas” (three sulfonamides formulated in solution together) allows increased efficacy without a significant increased risk of adverse effects (Bevill, 1988).
Mechanism of ActionSulfonamides rely on the requirement of susceptible organisms to synthesize folic acid as a precursor of other important molecular molecules in the cell. Sulfonamides act as false substrates in the synthesis of folic acid. Trimethoprim and ormetoprim (diaminopyrimidines, discussed in Section Potentiated Sulfonamides) produce a synergistic effect when used together by inhibiting the enzyme dihydrofolate reductase.
Folic acid metabolism is presented in Figure 32.2. Para-aminobenzoic acid (PABA), pteridines, glutamic acid, and the enzyme dihydropterate synthase interact to form dihydropteroic acid, the immediate precursor to dihydrofolic acid. Dihydropteroic acid is enzymatically converted to dihydrofolic acid by dihydrofolate synthase, followed by another enzymatic conversion of dihydrofolic acid to tetrahydrofolic acid (THFA) via dihydrofolate reductase (DHFR). The combination of sulfonamides and trimethoprim inhibits formation of tetrahydrofolic acid at two steps. This action is synergistic and increases activity against organisms that could otherwise be resistant. Tetrahydrofolate is a coenzyme in a number of complex enzymatic reactions and also is a coenzyme in the synthesis of thymidylic acid (a nucleotide), which is a building block of DNA. Trimethoprim and sulfonamides are bacteriostatic by themselves; together, they can be bactericidal. Bacteria are more susceptible to this combination than to either drug when tested alone (White et al., 1981).
Sulfonamides provide a false substrate for para-aminobenzoic acid (PABA) inhibiting the synthesis to dihydropteroic acid, a precursor for synthesis to dihydro- and tetrahydrofolic acid. Trimethoprim inhibits the enzyme dihydrofolate reductase, an enzyme critical to the synthesis of tetrahydrofolic acid.
Trimethoprim–sulfonamides are formulated in a ratio of 1:5 (trimethoprim:sulfonamide). In the animal, it is usually cited that the optimum ratio to produce antibacterial activity is 1:20 (Bushby, 1980; van Duijkeren et al., 1994b). Testing for susceptibility using approved CLSI methods (CLSI, 2015) uses a ratio of 1:20 trimethoprim:sulfonamide. However, this ratio is often much lower in animals because the trimethoprim component is excreted faster than the sulfonamide and the optimum ratio may actually be much wider than the value of 1:20 cited in human medical references, and may be as low as 1:40.
Sulfonamide action is dependent on the chemical similarity with PABA. Therefore, sulfonamides act as a false substrate in this reaction and synthesis of THFA is inhibited. The sulfonamides are relatively safe to mammalian cells because mammals utilize dietary folate for the synthesis of dihydrofolic acid, and they do not require PABA. The enzyme dihydrofolate reductase of bacteria has a much higher affinity (50,000 to 60,000-fold, and in some references as high as 100,000-fold) for trimethoprim than mammalian dihydrofolate reductase.
The mechanism of action of sulfonamides on bacteria does not entirely explain the activity against protozoa. Sulfonamides may inhibit protozoal dihydrofolate synthetase. Protozoal dihydrofolate reductase also is susceptible to the action of trimethoprim, which may explain some of the effect to support the use of these drugs for protozoal infections (treatment of protozoa infections is discussed in Chapter 42).
Clinical Uses and Microbial SusceptibilityThe spectrum of activity for the sulfonamides is broad, affecting gram-positive, gram-negative, and many protozoal organisms. Sulfonamides have been used clinically for approximately 50 years and many organisms once susceptible to the sulfonamides are now resistant. To increase the activity, most of the sulfonamides used in clinical practice are combinations with either trimethoprim or ormetoprim (diaminopyrimidines). These combinations (referred to in this chapter as trimethoprim–sulfonamides, but also referred to in clinical practice as trimethoprim–sulfa or simply abbreviated as TMP/SU) have increased the activity.
Administration of a single sulfonamide, or combination of sulfonamides, continues to be used in some livestock practices. In the United States, there are no approved formulations of trimethoprim–sulfonamides available for food animals, but trimethoprim–sulfadoxine is available in some countries.
The susceptibility/resistance patterns of sulfonamides and the trimethoprim–sulfamethoxazole combination against the most commonly encountered veterinary pathogens has been reported (van Duijkeren et al., 1994a, 1995; Bade et al., 2009; Winther et al., 2011). The activity of these agents has allowed for treatment of common respiratory infections, urinary tract and soft tissue infections, and intestinal infections (intestinal protozoa). Susceptible organisms include Arcanobacterium, Bacillus spp., E. rhusiopathiae, L. monocytogenes, Streptococcus spp., (Streptococcus equi subsp. zooepidemicus from horses), and protozoa (coccidia and Pneumocystis carinii).
The wild-type strains of following organisms are usually susceptible to the trimethoprim–sulfonamide (or ormetoprim–sulfonamide) combination: Pasteurella spp., Proteus spp., Salmonella spp., Histophilus (formerly Hemophilus), the protozoa Toxoplasma, and coccidia. Other bacteria that may be susceptible, but for which resistance can develop, include Staphylococcus spp., Corynebacterium, Nocardia asteroides, Stenotrophomonas maltophilia, and bacteria of the Enterobacteriaceae (Klebsiella, Proteus, Enterobacter, and Escherichia coli).
The organisms that are consistently resistant to trimethoprim–sulfonamide combinations include: Pseudomonas spp., Chlamydia spp., and Bacteriodes. One should cautiously interpret trimethoprim–sulfonamide susceptibility for Enterococcus spp. Although Enterococcus may appear susceptible to trimethoprim–sulfonamides using in vitro tests, it escapes the antifolate activity of the drug in vivo by its unique ability to incorporate preformed exogenous folates (Wisell et al., 2008). Sulfonamides alone are not active against Enterococcus spp. Clinical failures are reported despite in vitro susceptibility and microbiology laboratories should not report the susceptibilities of Enterococcus to trimethoprim–sulfonamides.
The activity of trimethoprim–sulfonamides against anaerobic bacteria can be variable. When measured in vitro, trimethoprim–sulfonamides have good activity against anaerobic bacteria (Indiveri and Hirsh, 1986), but clinical results are not as good (Dow, 1988) because thymidine and PABA (inhibitors of trimethoprim–sulfonamide activity) may be present in anaerobic infections.
Trimethoprim–sulfonamides have been used to treat infections caused by protozoa (including Toxoplasma gondii) and intestinal coccidia. Trimethoprim–sulfonamide combinations have also been used to treat equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona.
Interactions Affecting Antimicrobial ActivityComponents found in some tissue environments may inhibit trimethoprim–sulfonamide activity. For example, thymidine and PABA present in infected tissue – may interfere with activity. This has been demonstrated in tissue cages in horses. Ensink et al. (2005) showed an inability to eliminate the infection in an infected environment, despite in vitro sensitivity. They cited inhibitors – such as PABA and thymidine – present in abscessed and infected tissues that may inhibit the effects of these drugs. In another study in which trimethoprim–sulfadoxine was administered to cattle with infected tissue cages (Greko et al., 2002), it was shown that high levels of thymidine in the tissue cage fluid inhibited trimethoprim and compromised the ability to eradicate the infection.
Susceptibility TestingFor susceptibility testing, trimethoprim–sulfame- thoxazole (1:20 ratio of trimethoprim:sulfamethoxazole) should be used, even when trimethoprim–sulfadiazine is used for therapy (CLSI, 2013, 2015). There are no quality control (QC) ranges developed for trimethoprim–sulfadiazine, and tests using trimethoprim–sulfamethoxazole are expected to give equivalent results. Winther et al. (2011) showed that there were no significant differences observed between the minimal inhibitory concentration (MIC) of sulfadiazine and sulfamethoxazole for individual bacterial strains, confirming that sulfamethoxazole is an effective surrogate for susceptibility testing of sulfadiazine. The CLSI susceptibility testing standards state that Mueller–Hinton agar containing excessive amounts of thymidine or thymine can reverse the inhibitory effect of sulfonamides and of trimethoprim, which may result in false-resistant reports (CLSI, 2013). Susceptibility testing agar that is as thymidine free as possible should be used. The current CLSI interpretive categories (CLSI, 2015) do not provide veterinary-specific interpretations; therefore, the human breakpoint is used by laboratories to predict susceptibility. For Staphylococcus spp. and the Enterobacteriaceae the susceptible breakpoint is ≤2/38 (trimethoprim/sulfonamide) and for Streptococcus spp. the breakpoint is ≤0.5/9.5 (trimethoprim/sulfonamide).
Drug ResistanceResistance by many bacterial and protozoal organisms has become widespread due to the extensive use of sulfonamides over many years (Huovinen, 2001). Resistance occurs via efflux pumps, failure to penetrate the organism, and changes in target enzymes. Resistance can be transferable. Chromosomal resistance tends to occur slowly and confers resistance via impaired drug penetration into the microbial cell, producing an insensitive dihydropteroate enzyme and an increased production of PABA. Plasmid-mediated resistance, the most commonly encountered form of sulfonamide resistance, occurs quickly and manifests itself via the impaired drug penetration mechanism in addition to producing sulfonamide-resistant dihydropteroate synthase enzymes. If an organism becomes resistant to one sulfonamide, it is generally resistant to all other sulfonamides. Resistance to trimethoprim occurs via overproduction of the dihydrofolate reductase enzyme or synthesis of an enzyme that resists binding of the drug.
Oral AbsorptionIn dogs, absorption is excellent and not affected by feeding (Sigel et al., 1981). There has been considerable interest in the oral absorption of trimethoprim–sulfonamide combinations in horses and the effect of feeding. When trimethoprim-sulfonamides are administered to a horse that has not been fed, rapid absorption occurs, but is not as complete as for dogs or people. Nevertheless, oral administration is sufficient in horses to produce effective results. The fraction absorbed for trimethoprim was reported to be 67%, and for sulfadiazine 58%, but for both components the variability was high (van Duijkeren et al., 1994c). Oral absorption in another study in horses was 90.2% for intragastric administration and 74.45% for the oral paste (Winther et al., 2011). For trimethoprim in the same study, it was 71.5% oral absorption for the intragastric administration and 46% for the oral paste (Winther et al., 2011). In that study the absorption of trimethoprim–sulfadiazine was likely diminished by feeding. When trimethoprim–sulfadiazine was administered to horses as an oral suspension and compared to the equine paste, the absorption from the suspension was higher for both drugs compared to the paste, that is 136% and 118% of the paste AUC concentrations for sulfadiazine and trimethoprim, respectively (McClure et al., 2015). In another study (van Duijkeren et al., 1994c) the oral paste was compared to two compounded formulations (mixed with syrup and water or carboxymethylcellulose gel). In this comparison, all three formulations were judged to be equivalent. When administered to horses that have been fed or when it is added to the horses’ feed concentrate, a delayed and biphasic absorption is observed (van Duijkeren et al., 2002, 1995). When trimethoprim sulfachlorpyridazine was administered to horses, oral absorption was delayed, with the first peak appearing 1 hour after dosing and the second appearing 8–10 hours postdosing. Dual absorption peaks were not found after nasogastric administration (van Duijkernen et al., 1995). The best explanation for this phenomenon is that that there is an initial peak of absorption in the small intestine where much of drug absorption is known to occur. However, the drug that is bound to feed (adsorption) is unavailable for absorption until it travels to the cecum and, after digestion of the carbohydrates, the drug is released, producing a delayed and biphasic peak in absorption. Trimethoprim–sulfachlorpyridazine can bind to equine cecal contents 60–90%, which supports the theory of the “double peak”. Feeding also decreased the systemic availability from 70% when fasted to 45% when fed (van Duijkernen et al., 1996).
In ruminants, age and diet can markedly affect trimethoprim and oral sulfadiazine disposition in calves (Guard et al., 1986; Shoaf et al., 1987). Orally administered sulfadiazine (30 mg/kg) was absorbed very slowly in those calves fed milk diets, with absorption slightly higher in ruminating calves. Trimethoprim was absorbed in preruminant calves, but not absorbed in mature ruminants after oral administration (Shoaf et al., 1987), probably because of inactivation in the rumen.
Sulfasalazine is not used for the antibacterial properties, but is used to treat inflammatory disease of the large intestine in small animals (discussed in more detail in Chapter 46). It is not absorbed as a whole molecule but rather is cleaved into two more active compounds by native resident colonic bacteria.
DistributionSulfonamides distribute to most body fluids, but are not distributed to tissues as extensively as trimethoprim. Generally, sulfonamide tissue concentrations are lower than plasma concentrations (approximately 20–30% of corresponding tissue concentration), but distribution to extracellular fluids is generally high enough to produce effective concentrations against susceptible pathogens. High protein binding affects the distribution and markedly increases the half-life of sulfonamides.
Sulfonamides are weak acids and trimethoprim is a weak base. The ionization affects distribution, which favors the distribution and ion trapping of trimethoprim in tissues (intracellular environment is typically more negative than plasma). Therefore, trimethoprim has a higher volume of distribution than sulfonamides. Also, because sulfonamides are weak acids, the pH-partition hypothesis shows that these drugs do not attain therapeutic concentrations in milk; however, enough passive diffusion occurs to limit their use in dairy cattle.
Source: veteriankey
