Sulfamethoxazole (SMZ) is an antibacterial drug, which is a sulfonamide-based compound that is usually used in combination therapy to increase antibacterial effectiveness and the therapeutic area. The combination drug (and its co-formulation) with trimethoprim (TMP), commonly referred to as co-trimoxazole or trimethoprim/sulfamethoxazole (TMP/SMZ), can especially be used to manage a range of bacterial infections. TMP/SMZ's 1:15 ratio is designed to take advantage of the overlapping actions of both, allowing for a double inhibition of bacterial folate production (defective to DNA, RNA and protein synthesis). This mutually reinforcing inhibition vastly increases antibacterial activity against most germs and pathogens, including Gram-positive and Gram-negative bacteria, protozoa and some fungi.
Experimental Reagents
Sodium hydroxide, Potassium chloride, Ethanol, Sulfamethoxazole
| Catalog | Product Name | Order |
| ACM723466 | Sulfamethoxazole | Inquiry |
| ACM7447407 | Potassium chloride | Inquiry |
Analytical Methodology: Dual-Wavelength UV Spectrophotometry
The measurements of sulfamethoxazole and trimethoprim in compound samples can be conveniently made by dual-wavelength UV spectrophotometry. As both SMZ and TMP have different UV absorbances, they can be quantified together, without separating, by choosing wavelengths that will absorb both compounds most strongly.
Table 1 Analytical Conditions for UV Spectrophotometry of SMZ and TMP
| Compound | Medium | Measurement Wavelength (λ) | Reference Wavelength (λ) |
| Sulfamethoxazole | 0.1 mol/L NaOH solution | 257 nm | 304 nm |
| Trimethoprim | 0.0075 mol/L HCl ~ 0.1 mol/L KCl solution | 239 nm | 295 nm |
A. Sample Preparation: Tablets are crushed, weighed, dissolved in the selected solvent, and filtered to yield the test solution.
B. SMZ Measurement: Prepare the test solution and standard reference solution in a 0.1 mol/L NaOH medium. Measure absorbance at 257 nm (λ1) for quantification and 304 nm (λ2) for baseline correction, enabling precise determination of ΔA and SMZ concentration.
C. TMP Measurement: Similarly, prepare TMP test and standard solutions in a 0.0075 mol/L HCl ~ 0.1 mol/L KCl solution. Measure absorbance at 239 nm (λ3) for quantification and 295 nm (λ4) for baseline correction. Quantification is achieved by calculating ΔA, which is linearly related to TMP concentration.
Antibacterial Spectrum and Mechanism of Action
Sulfamethoxazole and trimethoprim share extensive antibacterial activity as it has a potent mechanism - in fact, it shuts down the metabolism of bacterial folate by two separate enzymes. sulfamethoxazole inhibits dihydropteroate synthase which disrupts folate formation in the first step, and trimethoprim inhibits dihydrofolate reductase which disrupts folate formation in the second step. This two-stage interference disrupts bacterial DNA and protein manufacturing to generate a bactericidal activity very potent against common pathogens like Haemophilus influenzae, Neisseria gonorrhoeae, Shigella spp., and Escherichia coli, among others.
Fig.1 Mechanism of action of trimethoprim/sulfamethoxazole[1].
Its dual-action design can be further adapted to specific protozoa such as Toxoplasma gondii and opportunistic pathogens such as Pneumocystis jirovecii. The lipophilicity of trimethoprim allows it to reach more thoroughly into tissues with acidic environments (like prostatic and vaginal secretions) and thus has better local virulence against bacteria.
Clinical Applications and Safety Profile
TMP/SMZ can be used for treating all kinds of infections such as urinary tract infections, respiratory tract infections, gastroenteritis and some types of protozoal infections. It is also a prophylactic and treatment for the prevention of opportunistic infections in immunocompromised people (especially, HIV/AIDS patients). But you must be vigilant because TMP/SMZ side effects are possible. Negative reactions include hypersensitivity (rashes, edema, dyspnoea) and kidney issues (like crystalluria, which can affect kidney function). The gastrointestinal tract is affected too and in very uncommon cases a severe reaction such as acute disseminated encephalomyelitis might occur.
Reference
- Trimethoprim-Sulfamethoxazole. Mayo Clinic Proceedings (1987).
