Susceptibility Testing of Biofilm to Guide Treatment of Periprosthetic Joint Infections

In the infectious process, biofilms may form early on biomaterials. There is ample evidence to the several-fold increase in antimicrobial concentrations required to eradicate even immature biofilms. In clinical practice the choice of antimicrobials is guided by susceptibility testing based on minimum inhibitory concentrations (MIC), which poorly reflect the increased antimicrobial resistance in biofilms. Minimum biofilm eradicating concentrations (MBEC) might better correspond to in vivo efficacy in the treatment of PJI, and other biofilm-centred infections. There is a need to develop methods, which are reproducible, low-cost, and time-efficient, for in vitro susceptibility testing of clinical biofilm bacteria.

This prospective randomized study aim to compare antibiotic treatment regimens for PJI guided by MIC or MBEC combined with MIC, and how it affects infection resolution, drug tolerability and relapse strain resistance patterns.

The investigors hypothesize that a standardised surgical debridement, antibiotics and implant retention (DAIR) for PJI followed by antibiotic guiding MBEC-diagnostics on deep tissue specimens will increase treatment efficacy and decrease the incidence of infection relapses compared to standard of care.

Following standardised debridement and 14 days of parenteral antibiotics; cloxacillin for methicillin sensitive staphylococci or vancomycin for methicillin resistant staphylococci, patients will be randomized to receive oral antibiotic combinations with either cell wall active standard of care antibiotic combination (MIC-guided) for 6 weeks or non cell wall active antibiotic combination according to a MIC- and MBEC-guided decision algorithm for 6 weeks.

Patients will be included at the Orthopaedic Infection Centre (OIC), Sahlgrenska University Hospital/Mölndal, which is a unit dedicated to optimizing management of orthopaedic infections. Patient consent acquisitions and randomisations (20 patients in each group) will be performed during post-operative hospitalisation. MIC determination and disk diffusion will be performed at the SWEDAC (Swedish Accreditation Body) certified clinical bacteriological laboratory. At the department of Biomaterials (University of Gothenburg), a previously developed clinical diagnostic tool will be employed for the MBEC determination. It consists of the combination of the Calgary Biofilm Device (MBECTM P\&G Assay, Innovotech) and a custom-made susceptibility plate (Substrata department, Sahlgrenska Hospital) with 6 antimicrobial agents commonly used to treat orthopaedic infections.

Inclusion and exclusion criteria are described in section "Eligibility".

The primary and secondary endpoints are described in section "Outcome measures".

Treatment criteria: for all administered antimicrobials staphylococcal strains must be susceptible in disc diffusion tests/MIC, regardless of MBEC-level. Antibiotic combinations will be selected from 5 already recommended non-cell wall active anti-staphylococcal antibiotics with high per-oral bio-availabilities and acceptable bone penetration used in the treatment of PJIs: rifampicin (RIF), fusidic acid (FUS), ciprofloxacin (CIP)/levofloxacin (LEV) and clindamycin (CLI).

Clinical breakpoints are expressed as MIC (EUCAST and CLSI) but are based on more than MIC distributions (epidemiological cut-off) namely:

• Pharmacokinetic and pharmacodynamic analyses of the antibiotic.
• Relation between MIC and probability of cure in clinical trials.

If likely effective according to EUCAST clinical breakpoints, antibiotics are further ranked in the MBEC treatment algorithm by bone penetration/susceptibility range and semi-arbitrarily by clinical efficacy in prosthetic joint infections.

MBEC/MIC cut-off for replacing Rifampicin despite susceptibility according to MIC:

• C-Max (oral dose of 750 mg) 10 mg/L x 0.4 (Bone/Serum) = 4 mg/L
• RIF-MIC clinical breakpoint for staphylococci is 0.5 mg/L.
• Clinical efficacy factor 1.
• MBEC/MIC ≥ 8 times MIC.

MBEC/MIC cut-off for replacing Levofloxacin despite susceptibility according to MIC:

• C-Max (oral dose of 750 mg) 12 mg/L x 0.6 (Bone/Serum) = 7,2 mg/L
• LEV-MIC breakpoint for staphylococci is 1 mg/L
• Clinical efficacy factor 0,75.
• MBEC/MIC ≥ 5 x MIC.

MBEC/MIC cut-off for not choosing Fusidic acid as companion drug:

• C-Max (500 mg) 30 mg/L x 0,2 (Bone/Serum) = 6 mg/L
• FUS-MIC breakpoint for staphylococci is 1 mg/L.
• Clinical efficacy factor 0,5.
• MBEC 3 times the MIC or more if better companion drug.

MBEC/MIC cut-off for not choosing Clindamycin as companion drug:

• C-Max (600 mg) 12 mg/L x 0.3 (Bone/Serum) = 3,6 mg/L
• CLI-MIC breakpoint for staphylococci is 0.5 mg/L
• Clinical efficacy factor 0,5.
• MBEC/MIC 4 x MIC or more if better companion drug.

MBEC/MIC cut-off for not choosing Linezolid (LIN) as sole drug:

• C-Max 21 mg/L x 0.4 (Bone/Serum) = 8,4 mg/L
• LIN-MIC breakpoint for staphylococci is 4 mg/L
• Clinical efficacy factor 0,5.
• MBEC 2 times the MIC or more.

MBEC cut-off for not choosing Sulfamethoxazole (SMX)/Trimethoprim (TMP) as sole drug:

• C-Max (3200/640 mg SMX/TMP) is 145 and 7.5 mg/L resp. x 0.25 (Bone/Serum) = 36 and 1,9 mg/L resp.
• SMX/TMP-MIC breakpoint for staphylococci is 2 mg/L. (By EUCAST expressed as the Trimethoprim konc.)
• Clinical efficacy factor 0,5.
• MBEC > MIC
• *Interpret as better than according to justification above.

All follow-up up to one year will be done according to clinical routines.