Tissue Penetration of Oral Bactrim

• Bactrim is rapidly absorbed following oral administration (85%)
• Peak blood levels for the individual components occur 1 to 4 hours after oral administration. The mean serum half-lives of sulfamethoxazole and trimethoprim are 10 and 8 to 10 hours, respectively
• Both sulfamethoxazole and trimethoprim distribute to sputum, vaginal fluid and middle ear fluid
• Average percentage of the dose recovered in urine from 0 to 72 hours after a single oral dose of sulfamethoxazole and trimethoprim is 84.5% for total sulfonamide and 66.8% for free trimethoprim
• trimethoprim also distributes to bronchial secretion
• both pass the placental barrier and are excreted in human milk.

Skin and Tissue Penetration

• Good penetration and are indicated for treatment of several infections
• pharmacokinetic/pharmacodynamic study found that trimethoprim/sulfamethoxazole exhibits good skin/soft tissue penetration in patients with Diabetic foot ulcers as well as bactericidal activity in serum against strains of S. aureus and β-haemolytic streptococci
• Oral options for treatment of gram-negative (osteomyelitis)  infections include fluoroquinolones and trimethoprim-sulfamethoxazole
• Has  excellent oral bioavailability that has been used to treat MSSA and MRSA bone infections, though most of the published experience is with courses of longer than 6 weeks and in device-associated infections. 
• Optimal dosing should be weight based: 5 to 10 mg/kg of trimethoprim/day divided into 2 or 3 doses in individuals with normal renal function

References:

1. http://www.aliem.com/sulfamethoxazole-trimethoprim-ssti-1-2-tablets-bid/
2. http://www.ncbi.nlm.nih.gov/pubmed/23873647
3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2884905/
4. http://emedicine.medscape.com/article/1348767-treatment