Sulfonamides and Trimethoprim- Sulfamethoxazole (TMP-SMX)

Russell E. Lewis

2026-07-20

Sulfonamides and Trimethoprim-Sulfamethoxazole: TMP-SMX

Russell E. Lewis, Pharm.D
Associate Professor of Infectious Diseases (MEDS-10/B)

russelledward.lewis@unipd.it
https://github.com/Russlewisbo
Slides and course materials: www.idpadova.com

Learning objectives (1 of 2)

By the end of this lecture, you should be able to:

1. Describe the history and development of sulfonamides
2. Explain the chemical structure-activity relationships of sulfonamides
3. Describe the mechanism of action of sulfonamides and trimethoprim
4. Explain the basis for synergy in TMP-SMX combination

Learning objectives (2 of 2)

5. List the major mechanisms of resistance to sulfonamides and trimethoprim
6. Describe the pharmacokinetics and tissue distribution
7. Recognize common and serious adverse reactions
8. Identify appropriate clinical indications for TMP-SMX
9. Manage drug interactions and special populations

PART 1: History and Discovery

The birth of antimicrobial chemotherapy

• 1932: Gerhard Domagk discovers Prontosil
• First synthetic antimicrobial agent
• German dye industry origin
• Protected mice from Streptococcus pyogenes

Domagk was working for I.G. Farbenindustrie when he discovered that the red dye Prontosil protected mice from streptococcal infections. He received the Nobel Prize in 1939, though the Nazi regime forced him to decline it initially.

Prontosil to sulfanilamide

• Prontosil was a prodrug
• Active metabolite: sulfanilamide
• Released in vivo through azo bond cleavage
• First use in US: July 1935
  • 10-year-old girl with H. influenzae meningitis
  • Unfortunately unsuccessful (too late in disease)

The French team of Tréfouël discovered that the active compound was actually sulfanilamide, not the parent dye. This was important because sulfanilamide couldn’t be patented, leading to widespread availability.

Evolution of sulfonamides

Decade	Development
1930s	Basic sulfanilamide modifications
1940s	Sulfadiazine for systemic infections
1950s	Trimethoprim synthesis (Hitchings)
1960s	TMP-SMX combination introduced
1970s+	Prophylaxis for opportunistic infections

Sulfonamides were the dominant antimicrobials before penicillin became widely available. Even today, in combination with trimethoprim, they remain clinically valuable.

PART 2:
Sulfonamide Chemistry and Structure

Sulfonamide structure basics

Key structural features:

• Benzene ring core
• Sulfonamide group (-SO₂NH₂)
• Free amino group at 4-carbon
• Similar to PABA

Sulfonamide structures

The free amino group at the 4-carbon position is essential for activity.The illustration shows the structural formulas of six different sulfonamide compounds labeled A through F. Panel A displays the structure of sulfanilamide, which consists of a benzene ring substituted with å primary amine and å sulfonamide group. Panel B presents para-aminobenzoic acid with carboxylic acid and an amine group in para positions on å benzene ring. Panel illustrates sulfadiazine, containing a sulfanilamide core with a pyrimidine ring attached through å nitrogen-sulfur finkage. Panel D features sulfisoxazole, with the sulfanilamide båse attached to a methyl-substituted isoxazole ring. Panel E shows sulfamethoxazole, where a methoxazole ring is bonded to the sulfanilamide group. Panel F presents sulfathalidine, also called bhthalvisulfathiazole, which contains a sulfanilamide moiety linked to a thiazole ring and a phthalyl group bearina a carboxylic acid.

Structure-activity relationships

Activity-enhancing modifications:

• Free 4-amino group → Essential for activity
• Sulfonyl (SO₂) substitutions → Increased PABA inhibition
• Examples: sulfadiazine, sulfisoxazole, sulfamethoxazole

Activity-decreasing modifications:

• N-1 substitutions → Decreased GI absorption

The nature of substitutions determines not only activity but also pharmacokinetic properties like absorption, solubility, and protein binding.

Classification of sulfonamides

Class	Examples	Key Features
Short/medium-acting	Sulfisoxazole, SMX	Most common, systemic
Long-acting	Sulfadoxine	T½ 100-230 hrs, malaria
GI-limited	Sulfasalazine	Poorly absorbed, IBD
Topical	Silver sulfadiazine	Burns, wounds

This classification is clinically useful. Short-acting agents are the most commonly used. Long-acting agents fell out of favor due to Stevens-Johnson syndrome risk but remain useful for malaria.

Sulfamethoxazole: The most important

• N’-(5-methyl-3-isoxazolyl) sulfanilamide
• Less soluble than sulfisoxazole
• Higher blood levels achieved
• The sulfonamide in TMP-SMX
• Half-life ~11 hours (matches TMP)

Sulfamethoxazole was chosen for the TMP combination specifically because its pharmacokinetic profile (particularly half-life) closely matches that of trimethoprim, making it ideal for twice-daily dosing.

PART 3: Mechanism of Action

The folic acid pathway

Step 1: Sulfonamide action

Target: Dihydropteroate synthase (DHPS)

• Sulfonamides are PABA analogs
• Competitive inhibition of PABA incorporation
• Can be incorporated into dihydropteroate
• Result: Decreased dihydrofolic acid synthesis

Sulfonamides may actually have higher affinity for DHPS than the natural substrate PABA. They may also be incorporated into abnormal folate intermediates.

Step 2: Trimethoprim action

Target: Dihydrofolate reductase (DHFR)

• Blocks conversion of dihydrofolate → tetrahydrofolate
• 50,000-100,000x more active against bacterial vs human DHFR
• Selective toxicity is key
• Result: Decreased tetrahydrofolic acid

The remarkable selectivity of trimethoprim for bacterial DHFR over human DHFR is the basis for its therapeutic index. Human cells obtain folic acid from the diet and don’t require the synthesis pathway.

Sequential blockade = synergy

Why synergy occurs: Dual pathway blockade, bacteriostatic → Bactericidal effect, reduced resistance emergence

Key concept: Bacteriostatic vs bactericidal

Property	Sulfonamide Alone	TMP Alone	TMP-SMX
Effect	Bacteriostatic	Bacteriostatic	Bactericidal
Inhibition	DHPS	DHFR	Both
Resistance	Higher risk	Higher risk	Lower risk

The synergistic combination achieves bactericidal activity against many organisms that either agent alone would only inhibit. This is clinically relevant for serious infections.

Why humans are spared

Selective toxicity explained:

1. Humans cannot synthesize folic acid
2. Humans obtain folate from diet
3. Human DHFR has very low affinity for TMP
4. Mammalian cells take up preformed folate

Tip

High doses or prolonged therapy can still cause folate deficiency - supplement with leucovorin when needed

This is why TMP-SMX is generally well-tolerated despite blocking such fundamental metabolic pathways. However, prolonged high-dose therapy can deplete folate stores, especially in malnourished patients.

PART 4: Antimicrobial Spectrum and Resistance

Spectrum of activity

Gram-positive:

• S. aureus (including many CA-MRSA)
• S. pneumoniae (resistance increasing)
• Group A, B Streptococci (classically throught to be resistant-artifact of thymidine in test medium)
• Listeria monocytogenes
• Nocardia species

Gram-negative:

• Most Enterobacterales
• H. influenzae
• Stenotrophomonas maltophilia
• NOT Pseudomonas aeruginosa

TMP-SMX has an impressively broad spectrum. Notable gaps include Pseudomonas and most anaerobes. The activity against MRSA and Stenotrophomonas is particularly valuable clinically.

Activity against special pathogens

Organism	Activity	Clinical Use
Pneumocystis jirovecii	+++	First-line
Toxoplasma gondii	++	Alternative
Nocardia spp.	+++	First-line
Stenotrophomonas	+++	First-line
Isospora/Cyclospora	+++	First-line

These special pathogens are where TMP-SMX really shines. For PCP and Nocardia, it’s the drug of choice. For Stenotrophomonas, it’s often the only oral option.

Resistance: The growing problem

Resistance rates are increasing:

• E. coli (UTI isolates): 20-30% resistant in many areas
• S. pneumoniae: 25-50% resistant globally
• Shigella: >30% resistant in US, >75% in China
• Salmonella: Majority of isolates now resistant

Warning

Always check local resistance patterns before empiric therapy!

The once-reliable activity of TMP-SMX against common pathogens is being eroded by resistance. This is why it’s no longer first-line for many UTIs in high-resistance areas.

Mechanisms of resistance-sulfonamides

Multiple mechanisms can operate simultaneously. The sul genes are particularly concerning as they’re often carried on mobile genetic elements like integrons.

Mechanisms of resistance-trimethoprim (continued)

Important

Cross-resistance between sulfonamides is common; resistance to one = resistance to all

The large number of dfr gene variants (over 30 identified) reflects the strong selective pressure that has been applied to these drugs over decades of use.

PART 5: Pharmacology

Pharmacokinetics overview

Parameter	TMP	SMX
Bioavailability	>90%	>90%
Tmax	1-4 hr	1-4 hr
Half-life	8-10 hr	9-11 hr
Protein binding	45%	70%
CSF penetration	40-50%	25-50%

The matched pharmacokinetics of TMP and SMX make them ideal combination partners. Both have excellent oral bioavailability and penetrate well into tissues.

The magic of the 1:5 ratio

Fixed-dose combination:

• TMP 160 mg + SMX 800 mg = DS tablet
• Produces serum ratio ~1:20
• Optimal for synergy against most pathogens

Why this ratio?

• Accounts for different protein binding
• Accounts for different tissue distribution
• Maximizes bactericidal synergy

The 1:5 ratio in the tablet produces approximately a 1:20 ratio in serum due to the different protein binding and distribution characteristics. This was carefully optimized in early clinical trials.

IV to oral conversion & formulations

~90-100% oral bioavailability → convert IV to PO 1:1 (based on TMP)

Formulation (EU / Italy)	TMP + SMX per unit	Common label / pack
IV infusion (fiala)	16/80 mg per mL; 5 mL ampoule = 80/400	“480 mg”
Tablet — standard (compresse)	160/800 mg	“Bactrim forte” / 960 mg
Tablet — single strength	80/400 mg	“480 mg”
Oral suspension (Italy)	80/400 mg per 5 mL	100 mL bottle

Convert when: tolerating oral intake, hemodynamically stable, GI absorption intact

Always dose by the TMP component

TMP is the dose-limiting, dose-defining moiety. Prescribe and calculate mg/kg on TMP, not the combined product or the SMX. e.g. 15 mg/kg/day TMP for severe infections, divided q6-8h — the SMX follows the fixed 1:5 ratio.

Warning

EU vs US labelling trap: European/Italian products are labelled by the combined amount (“480 mg” = 80 TMP + 400 SMX), while US references dose by TMP alone. Always specify the TMP dose to avoid 5-fold errors.

Worked example: dosing by TMP

70 kg patient, Stenotrophomonas maltophilia pneumonia, normal renal function

1. Choose target: 10 mg/kg/day of TMP → 70 × 10 = 700 mg TMP/day
2. Divide q8h → ≈ 240 mg TMP per dose (700 ÷ 3, rounded to whole ampoules)
3. Express in product units:

Route	Per dose (q8h)	In “480 mg” units
IV	3 ampoules (3 × 80 = 240 mg TMP)	3 × 480 mg = 1440 mg
PO	1.5 standard tablets (160 mg TMP each)	1.5 × 960 mg

Oral step-down uses the same TMP dose — no reduction for the switch.

Note

Note how the same regimen reads as “240 mg TMP q8h” (US style) or “1440 mg co-trimoxazole q8h” (EU style). Anchor on the TMP number and the two never conflict.

Per il contesto italiano

La scheda tecnica italiana (Bactrim perfusione) esprime la posologia sul totale cotrimossazolo, tipicamente come mg/kg di entrambi i componenti (es. 20 mg/kg/die di trimetoprim + 100 mg/kg/die di sulfametossazolo, in 3–4 fiale ogni 6 ore). In reparto ordinate in fiale da “480 mg” (80 mg TMP), ma calcolate e verificate sempre la dose sul trimetoprim per evitare errori di un fattore 5.

Tissue distribution

Excellent penetration into:

• Cerebrospinal fluid (40-50%)
• Prostatic tissue
• Respiratory secretions
• Middle ear fluid
• Synovial fluid
• Pleural and peritoneal fluids

Tip

Good CNS penetration makes TMP-SMX useful for: - Toxoplasmic encephalitis - Nocardia brain abscess

The good tissue penetration, especially into CSF, is a major advantage. Many antibiotics do not penetrate the CNS well, limiting their usefulness for these serious infections.

Metabolism and elimination

Trimethoprim:

• 50-70% excreted unchanged in urine
• Hepatic metabolism (minor)
• Active tubular secretion

Sulfamethoxazole:

• Hepatic acetylation and glucuronidation
• 15-30% excreted unchanged in urine
• Metabolized by CYP2C9

Both components have significant renal excretion, which is why dose adjustment is needed in renal impairment. The high urinary concentrations also explain the effectiveness for UTIs.

Renal dosing adjustments

CrCl (mL/min)	Dose Adjustment
>30	Full dose
15-30	50% reduction
<15	Avoid (or 50% q24h with monitoring)

Hemodialysis: Give dose after dialysis

Warning

Monitor creatinine closely - TMP can increase serum creatinine by blocking tubular secretion (not true nephrotoxicity)

The creatinine increase from TMP is a pharmacokinetic effect, not true nephrotoxicity. TMP blocks the tubular secretion of creatinine, raising serum levels without affecting true GFR. However, actual nephrotoxicity (interstitial nephritis) can also occur.

PART 6: Adverse Effects

Common adverse effects

Gastrointestinal (most common):

• Nausea, vomiting
• Anorexia
• Diarrhea

Dermatologic:

• Rash (3-5% general population)
• Much higher in HIV (50-60%)
• Usually maculopapular

GI side effects are the most common reason for discontinuation in clinical practice. Taking with food may help. The dramatically higher reaction rate in HIV patients is important to anticipate.

Life-threatening reactions

1. Stevens-Johnson Syndrome / TEN
   • Mortality up to 30-40% for TEN
   • Usually within first 8 weeks
2. Severe hematologic toxicity
   • Agranulocytosis
   • Aplastic anemia
   • Thrombocytopenia
3. Anaphylaxis

SJS/TEN is the most feared complication. Risk factors include HIV infection and certain HLA types. If a patient develops a severe rash, stop the drug immediately and never rechallenge.

Hyperkalemia: An underappreciated risk

Mechanism:

• TMP blocks ENaC sodium channel
• Acts like potassium-sparing diuretic
• Occurs at therapeutic doses

Risk factors:

• Renal insufficiency
• Age >65 years
• ACE inhibitors or ARBs
• Higher TMP doses
• Diabetes mellitus

Hyperkalemia from TMP-SMX is more common than many clinicians realize. Multiple studies have shown increased sudden death risk in patients on ACEi/ARBs who receive TMP-SMX, likely due to hyperkalemia.

Hyperkalemia management

Potassium Level	Action
<5.5 mEq/L	Monitor
5.5-6.0 mEq/L	Recheck, consider dose reduction
6.0-6.5 mEq/L	Stop TMP-SMX, dietary K+ restriction
>6.5 mEq/L	Aggressive treatment, alternative antibiotic

Tip

Check potassium within first week in high-risk patients!

Routine potassium monitoring is especially important in patients on ACE inhibitors, ARBs, or with renal impairment. Most cases of significant hyperkalemia occur within the first 7-10 days.

Hematologic toxicity

Folate-related:

• Megaloblastic anemia
• Leukopenia
• Thrombocytopenia
• Risk increases with duration

Prevention:

• Supplemental leucovorin (folinic acid) for high-dose/prolonged therapy
• Especially important in malnourished patients

The hematologic effects are related to folate depletion. They’re usually reversible with drug discontinuation. Leucovorin can bypass the folate synthesis block without reducing antibacterial efficacy.

Special population: HIV patients

Much higher adverse reaction rates:

• Rash: 50-60% (vs 3-5%)
• Fever common
• Often occurs after 1-2 weeks
• May tolerate rechallenge or desensitization

Despite reactions, TMP-SMX remains:

• First-line PCP prophylaxis
• First-line PCP treatment
• Benefits outweigh risks

The high reaction rate in HIV patients was noted early in the AIDS epidemic. The mechanism is unclear but may relate to altered drug metabolism, immune dysregulation, or increased oxidative stress.

Pregnancy considerations

Avoid in late pregnancy

• Sulfonamides compete for bilirubin binding sites
• Risk of neonatal hyperbilirubinemia
• Kernicterus risk in newborn
• Also avoid in breastfeeding

First trimester:

• Some studies suggest neural tube defect risk
• Consider folate supplementation if used

The kernicterus risk is specific to the neonatal period when the blood-brain barrier is immature and bilirubin encephalopathy can occur. TMP-SMX should be avoided after 32 weeks of pregnancy.

PART 7: Drug Interactions

Major drug interactions

Drug	Effect	Mechanism
Warfarin	↑ INR	CYP2C9 inhibition
Methotrexate	↑ Toxicity	Protein displacement, DHFR inhibition
Phenytoin	↑ Levels	CYP2C9 inhibition
Sulfonylureas	↑ Hypoglycemia	Protein displacement

The warfarin interaction is probably the most clinically important. Always reduce warfarin dose when starting TMP-SMX and monitor INR closely.

Drug interactions (continued)

Drug	Effect	Management
ACEi/ARBs	↑ Hyperkalemia	Monitor K+
Dofetilide	↑ QT prolongation	Contraindicated
Cyclosporine	↓ Levels	Monitor
Digoxin	↑ Levels	Monitor

Important

TMP-SMX + Dofetilide = Absolute contraindication

Dofetilide is eliminated by renal cation transporters that TMP inhibits, leading to dangerous accumulation and QT prolongation. This is a hard contraindication.

PART 8: Clinical Applications

PCP: The most important indication

Pneumocystis jirovecii Pneumonia:

• Treatment: 15-20 mg/kg/day TMP, 21 days
• Prophylaxis: 1 DS tablet daily (or 3x/week)
• First-line for both
• Add steroids if PaO₂ <70 mmHg

Prophylaxis indications:

• HIV with CD4 <200 cells/µL
• Other immunocompromising conditions
• Solid organ transplant recipients

TMP-SMX prophylaxis dramatically reduced PCP mortality in the HIV epidemic. Today it remains essential for immunocompromised patients. The 3x weekly dosing is often better tolerated with similar efficacy.

Urinary tract infections

Uncomplicated cystitis:

• 1 DS tablet BID × 3 days
• Only if local resistance <20%

Pyelonephritis:

• 1 DS tablet BID × 7-14 days
• If susceptible

Warning

Check your local antibiogram! Many areas now have E. coli resistance >20%

TMP-SMX was once first-line for UTIs everywhere. Now, due to resistance, IDSA guidelines recommend it only in areas where resistance remains below 20%. Fluoroquinolones or nitrofurantoin/fosfomycin may be preferred.

Skin and soft tissue infections

Community-acquired MRSA:

• TMP-SMX has good CA-MRSA activity
• 1-2 DS tablets BID × 5-10 days
• Often combined with I&D for abscesses
• Useful oral step-down option

Advantages:

• Oral bioavailability
• Good tissue penetration
• Often susceptible when other agents fail

TMP-SMX has become a key agent for CA-MRSA skin infections because it’s oral, well-tolerated, and usually effective.

The myth about TMP/SMX and lack of activity against Streptococci

• The key issue was thymidine in culture media.

• Streptococci can utilize exogenous thymidine.

  • Early susceptibility media often contained relatively high thymidine concentrations.

  • Because TMP/SMX inhibits tetrahydrofolate-dependent thymidylate synthesis, providing thymidine externally bypasses the drug’s mechanism.

• The result was falsely elevated MICs and apparent resistance.

• Modern CLSI-recommended media contain low thymidine concentrations (or thymidine phosphorylase is added), largely eliminating this artifact.

How do “appropriate” test conditions of TMP/SMX impact Streptococcus susceptibility?

Most isolates of:

• Streptococcus pyogenes (Group A Streptococcus)
• Streptococcus agalactiae (Group B Streptococcus) many β-hemolytic streptococci

…are actually susceptible in vitro when tested appropriately.

Nocardiosis

First-line therapy:

• TMP-SMX preferred
• High doses: 15-20 mg/kg/day TMP
• Duration: 6-12 months (or longer)
• May combine with other agents for severe disease

Alternative: Imipenem, amikacin, linezolid

Nocardiosis requires prolonged therapy due to the organism’s slow growth. For CNS disease, even longer courses may be needed. TMP-SMX penetrates well into brain abscesses.

Stenotrophomonas maltophilia

Often the only oral option:

• Intrinsic multidrug resistance
• TMP-SMX usually active
• Ceftazidime-avibactam + aztreonam or cefidericol may be preferrable in some populations
• Important for step-down therapy

Tip

Think about Stenotrophomonas in: - ICU patients on broad-spectrum antibiotics - Ventilator-associated pneumonia - Malignancy patients

Stenotrophomonas is notoriously resistant to carbapenems and many other agents. TMP-SMX activity is a major advantage, especially for converting IV therapy to an oral option.

Stenotrophomonas: dosing & the combination-therapy shift

Current dosing (non-cystitis)

• 8–12 mg/kg/day TMP, divided q8–12h
• Consider a max of ~960 mg/day TMP
• Cystitis: lower doses acceptable (high urinary levels)

The recent shift (Tamma et al. 2024)

• IDSA guidance now favors TMP-SMX as part of combination therapy, at least until clinical improvement
• Reflects doubt that an “S” on the report predicts monotherapy success

Why the caution? (Lasko et al. 2022)

• Breakpoints largely inherited from other species
• In vitro PK/PD: even ~20 mg/kg/day TMP failed to achieve stasis; up to the equivalent of 100 mg/kg/day could not reach 1-log kill
• Derived free AUC/MIC stasis target ≈ 67 — pushing the dose does not improve target attainment
• Murine models poorly translational (high rodent thymidine)

Important

First ask: colonizer or true pathogen? Steno agents are limited and often poorly tolerated — avoid treating culture results rather than disease.

Clinical data are heavily confounded (“die from vs with steno”), mostly retrospective, frequently without MICs, and usually involve combination therapy — so monotherapy efficacy remains uncertain. Alternatives (e.g. levofloxacin, cefiderocol, ceftazidime-avibactam plus aztreonam) are also imperfect, making this a genuinely difficult treatment landscape.

Other clinical uses

Infection	Dose	Duration
Toxoplasmosis	High-dose	6+ weeks
Traveler’s diarrhea	1 DS BID	3-5 days
Isosporiasis	1 DS QID	10 days
Cyclosporiasis	1 DS BID	7-10 days
Listeria meningitis	High-dose IV	3+ weeks

TMP-SMX remains useful for a variety of infections beyond the major indications. For listeria, it’s an important alternative for penicillin-allergic patients.

PART 9: Practical Prescribing

Dosing summary

Indication	Dose	Frequency	Duration
PCP prophylaxis	1 DS	Daily or 3x/wk	Indefinite
PCP treatment	15-20 mg/kg TMP	Q6-8h	21 days
Uncomplicated UTI	1 DS	BID	3 days
SSTI	1-2 DS	BID	5-10 days
Nocardiosis	15 mg/kg TMP	Divided	6-12 mo

This table provides a quick reference for common dosing regimens. Remember to adjust for renal function and monitor for adverse effects, especially with prolonged use.

Optimizing the dose: principles

Dose the trimethoprim component

• Fixed 1:5 TMP:SMX ratio for all formulations
• Express dose as mg/kg/day of TMP
• SS = 80 mg TMP; DS = 160 mg TMP

Indication drives the target

• GN bacteremia (step-down): ~5 mg/kg q12h
• CA-MRSA SSTI: ~5 mg/kg/day (1 DS BID most)
• Bone/joint: ~10 mg/kg/day
• PCP treatment: 15–20 mg/kg/day

Why “optimal” is hard

• No validated PK/PD index (unlike vanco AUC/MIC)
• Legacy targets extrapolated from 1970s–90s data
• Emerging PK data suggest lower PCP doses (<10 mg/kg/day) may retain efficacy with fewer ADEs (Butler-Laporte et al. 2020)
• IV formulation adds large fluid/D5W burden

Tip

Give an actual mg dose using the patient’s real weight rather than a weight-based formula — reduces transcription and math errors. Use the lowest effective dose and get creative with split dosing/timing with meals to improve tolerability.

Dosing schemes vary enormously by indication (a few tablets weekly to multiple tablets several times daily). Because there is no clear PK/PD target, recommendations lean heavily on package labeling, older PK literature, and retrospective step-down studies. For PCP, dose-limiting toxicity drives treatment failure in up to one-third of patients, which motivates interest in lower-dose strategies.

Possible role of therapeutic drug monitoring

Rationale

• Wide interpatient PK variability
• Narrow therapeutic margin at high doses (PCP)
• Toxicities are largely dose/exposure-dependent (hyperkalemia, marrow suppression, GI, rash)
• No consensus PK/PD index → TDM remains investigational

What is measured

• Sulfamethoxazole peak most commonly reported
• Historical PCP peak target ~100–150 µg/mL (SMX), 1–2 h post-dose (Dao et al. 2014)
• Modern POPPK: SMX peak >100 mg/L & TMP >5 mg/L (efficacy); toxicity above SMX >200, TMP >15 mg/L (Leegwater et al. 2025)
• Trimethoprim levels measured less often; assays not widely available

Where TDM may help most

• High-dose/prolonged PCP treatment with poor tolerability
• Altered PK: obesity, augmented renal clearance, CRRT/dialysis, critical illness
• Difficult pathogens (e.g. Stenotrophomonas) where exposure–response is uncertain

Warning

TDM for TMP-SMX is not standardized — assay access, validated targets, and outcome data are limited. Use alongside clinical response, renal function, potassium, and CBC rather than as a stand-alone endpoint.

The lack of a validated PK/PD index is the central barrier: without a clear exposure target linked to outcome, TDM cannot yet be applied the way it is for vancomycin or aminoglycosides. Even so, measuring exposure may help individualize therapy in patients at the extremes of PK or those failing to tolerate standard high doses — targeting the lowest effective exposure to preserve efficacy while limiting dose-dependent harm.

Desensitization protocols

When to consider:

• HIV patients needing PCP prophylaxis
• Previous mild-moderate reactions
• NOT for SJS/TEN history

Rapid 8-hour protocol:

• Escalating doses hourly
• Hospital setting with anaphylaxis capability
• Success rate ~70-80%

Desensitization can be life-saving for HIV patients who need PCP prophylaxis but have had prior reactions. It should only be done in a controlled setting by experienced clinicians.

Contraindications

Absolute:

• Known hypersensitivity to sulfonamides or TMP
• History of SJS/TEN with sulfonamides
• Megaloblastic anemia from folate deficiency
• Severe hepatic or renal impairment
• Pregnancy at term (>32 weeks)
• Infants <2 months (except PCP)

These contraindications should be checked before prescribing. The infant age restriction is due to immature bilirubin metabolism and kernicterus risk.

Monitoring recommendations

Parameter	Timing	Notes
CBC	Baseline, periodically	Cytopenias
Creatinine	Baseline, week 1	May ↑ from TMP
Potassium	Week 1	High-risk patients
LFTs	If prolonged use	Hepatotoxicity
INR	If on warfarin	Interaction

The extent of monitoring depends on the patient’s risk factors and duration of therapy. Short courses in healthy patients need minimal monitoring; long-term use requires more attention.

PART 10: Clinical Cases

Case 1: PCP Prophylaxis

45-year-old man with HIV:

• CD4 count: 180 cells/µL
• No prior opportunistic infections
• Taking ART with good adherence

Question: What prophylaxis do you recommend?

Case 1: Answer

TMP-SMX 1 DS tablet daily (or 3x weekly)

Key points:

• CD4 <200 = indication for PCP prophylaxis
• TMP-SMX is first-line
• Continue until CD4 >200 for 3+ months on ART
• Also provides protection against toxoplasmosis

TMP-SMX prophylaxis can be discontinued once CD4 counts have been >200 for at least 3 months on effective ART. The 3x weekly dosing may be considered if daily dosing causes GI intolerance.

Case 2: UTI in 2025

28-year-old woman with uncomplicated cystitis:

• Dysuria, frequency × 2 days
• No fever
• Local E. coli TMP-SMX resistance: 35%

Question: Is TMP-SMX appropriate?

Case 2: Answer

No - local resistance too high

Better options:

• Nitrofurantoin 100 mg BID × 5 days
• Fosfomycin 3 g single dose
• If fluoroquinolone indicated: short course

Rule: TMP-SMX only if local resistance <20%

This illustrates why knowing your local antibiogram is essential. TMP-SMX would have been first-line 20 years ago but is no longer appropriate in many areas.

Case 3: Hyperkalemia risk

72-year-old man with cellulitis:

• History: DM2, CKD (CrCl 35), HTN
• Medications: lisinopril, metformin, spironolactone
• Started TMP-SMX DS BID for CA-MRSA cellulitis

What’s the concern?

Multiple hyperkalemia risk factors: age, CKD, ACE inhibitor, and potassium-sparing diuretic!

Case 3: Answer

High hyperkalemia risk!

Risk factors present:

• Age >65 ✓
• CKD (CrCl 35) ✓
• ACE inhibitor ✓
• Spironolactone ✓

Management:

• Check baseline K+
• Recheck in 2-3 days
• Consider holding spironolactone during treatment
• Alternative antibiotic if K+ >5.5

This patient has essentially every risk factor for TMP-SMX induced hyperkalemia. Close monitoring and potentially holding the spironolactone would be prudent. Some might argue for a different antibiotic choice entirely.

Case 4: The rash

35-year-old HIV+ man on TMP-SMX prophylaxis:

• Day 10: develops diffuse maculopapular rash
• No mucosal involvement
• No systemic symptoms
• Tolerating oral intake

Options?

This is a common scenario. The key is distinguishing mild reactions that might be managed from severe reactions requiring immediate discontinuation.

Case 4: Answer

Options to consider:

1. Stop TMP-SMX and use alternative (dapsone, atovaquone)
2. Continue with antihistamines (mild reactions may resolve)
3. Plan for desensitization if alternative poorly tolerated

Red flags requiring immediate discontinuation:

• Mucosal involvement
• Systemic symptoms
• Blistering or desquamation
• Fever >38.5°C

Without red flags, some practitioners will continue through mild rashes with symptomatic treatment, as reactions sometimes resolve spontaneously. However, close monitoring is essential.

Case 5: Drug interaction

68-year-old woman on warfarin for A-fib:

• INR therapeutic at 2.5
• Started TMP-SMX for UTI
• Returns 5 days later with INR 5.8
• No bleeding

What happened?

Case 5: Answer

TMP-SMX inhibits CYP2C9 → ↑ warfarin levels

Management:

• Hold warfarin
• Vitamin K 1-2 mg PO if significant bleeding risk
• Recheck INR in 24-48 hours
• Resume warfarin at reduced dose
• Always reduce warfarin when starting TMP-SMX

This interaction is predictable and preventable. Best practice is to empirically reduce warfarin dose by 25-50% when starting TMP-SMX and monitor INR closely.

Summary and Key Takeaways

Key Points to Remember (1/2)

1. Mechanism: Sequential blockade of folate synthesis (DHPS + DHFR)

2. Synergy: Combination is bactericidal; components alone are bacteriostatic

3. Spectrum: Broad, but NOT Pseudomonas or anaerobes

4. Resistance: Increasing; always check local patterns for UTIs

Key Points to Remember (2/2)

5. PCP: First-line for both treatment and prophylaxis

6. Adverse effects: Higher in HIV; watch for SJS/TEN, hyperkalemia

7. Drug interactions: Warfarin, methotrexate, ACEi/ARBs

8. Contraindications: Late pregnancy, severe renal/hepatic impairment, SJS history

9. Dose optimization: Target the lowest effective TMP dose; emerging data support lower-dose PCP, and TDM (SMX peak) may help individualize therapy at PK extremes

Clinical application requires awareness of adverse effects and interactions. TMP-SMX remains valuable but must be used thoughtfully. The lowest-effective-dose principle and cautious use of TDM are increasingly relevant given dose-dependent toxicity.

When to Choose TMP-SMX

Excellent choice for:

• PCP (treatment and prophylaxis)
• Nocardiosis
• Stenotrophomonas
• CA-MRSA skin infections
• UTIs (if local susceptibility high)

Think twice if:

• High local resistance
• Multiple hyperkalemia risk factors
• Drug interactions (warfarin, MTX)
• Late pregnancy

References

Butler-Laporte, Guillaume, Elizabeth Smyth, Alexandre Amar-Zifkin, Matthew P. Cheng, Emily G. McDonald, and Todd C. Lee. 2020. “Low-Dose Trimethoprim-Sulfamethoxazole for the Treatment of Pneumocystis jirovecii Pneumonia: A Systematic Review and Meta-Analysis.” Open Forum Infectious Diseases 7 (5): ofaa112. https://doi.org/10.1093/ofid/ofaa112.

Dao, Betsy D., Jason N. Barreto, Robert C. Wolf, Ross A. Dierkhising, Matthew F. Plevak, and Pritish K. Tosh. 2014. “Serum Peak Sulfamethoxazole Concentrations Demonstrate Difficulty in Achieving a Target Range: A Retrospective Cohort Study.” Current Therapeutic Research 76: 104–9. https://doi.org/10.1016/j.curtheres.2014.08.003.

Lasko, Maxwell J., Matthew L. Gethers, Jennifer L. Tabor-Rennie, David P. Nicolau, and Joseph L. Kuti. 2022. “In Vitro Time-Kill and Pharmacodynamics of Trimethoprim-Sulfamethoxazole Against Stenotrophomonas maltophilia.” Antimicrobial Agents and Chemotherapy 66 (2): e0158321. https://doi.org/10.1128/AAC.01583-21.

Leegwater, Emiel, Lauren Baidjoe, Erik B. Wilms, et al. 2025. “Population Pharmacokinetics of Trimethoprim/Sulfamethoxazole: Dosage Optimization for Patients with Renal Insufficiency or Receiving Continuous Renal Replacement Therapy.” Clinical Pharmacology and Therapeutics 117 (1): 184–92. https://doi.org/10.1002/cpt.3421.

Tamma, Pranita D., Emily L. Heil, Julie Ann Justo, Amy J. Mathers, Michael J. Satlin, and Robert A. Bonomo. 2024. “Infectious Diseases Society of America 2024 Guidance on the Treatment of Antimicrobial-Resistant Gram-Negative Infections.” Clinical Infectious Diseases, ciae403. https://doi.org/10.1093/cid/ciae403.