Antimicrobial therapy in horses: a pharmacologist perspective

1. Antimicrobial therapy in horses: a pharmacologist perspective Pierre-Louis Toutain National Veterinary School; Toulouse ,France 30th October 2014; Department of Veterinary Disease Biology University of Copenhagen

2. Steps for a rationale selection of an antimicrobial (AM) drug • Identity of the affecting MO • In vitro AM susceptibility of the bug • Nature and site of infection • The pharmacokinetic (PK) characteristics of the selected AM • The pharmacodynamics (PD) properties of the selected AM • PK and PD integration (PK/PD indices) • Safety issues • Cost of the therapy

3. 1-Why plasma concentrations are relevant for AMD and why to compare free plasma concentration to MICs?

4. Nature and site of infectionWhere are located the pathogens Bound Extra Cellular Fluid Most bacteria of clinical interest - respiratory infection - wound infection - digestive tract inf. • Cell • (in phagocytic cell most often) • Legionnella spp • mycoplasma (some) • chlamydiae • Brucella • Cryptosporidiosis • Listeria monocytogene • Salmonella • Mycobacteria • Rhodococcus equi Free Free ±MIC MO

5. 2-The right dosage regimen to control the efficacious plasma concentration

6. What are the elements of a dosage regimen • The dose • A PK/PD variable • The dosing interval • The treatment duration • When to start • When to finish

7. A fundamental relationship PD X MIC PK PK (0 to 1) PK (0 to 1) ! A dose can be determined rationally using a PK/PD approach

8. Question: what is the daily dose for enrofloxacin for different possible MIC90 • What we know: • Plasma clearance: 2.5L/Kg/24h • Bioavailability by intragastric route of 80% • Extent of binding of ~ 20% • MIC90 • The PK/PD index for optimization: AUC/MIC=125 • Or equivalently : the average plasma concentration over the dosing interval should be 5 folds the MIC

9. It has been developed surrogates indices (predictors) of antibiotic efficacy taking into account MIC (PD) and exposure antibiotic metrics (PK) • Practically, 3 indices cover all situations: • AUC/MIC • Time>MIC • Cmax/MIC

10. Recommandations thérapeutiques en fonction de la bactéricide

11. The dose for enrofloxacin

12. The dose for enrofloxacinAUC/MIC=125

13. 3-Variability of plasma clearance in horses Drugs, age

14. AMD: plasma clearances Low or high?

15. AMD: plasma clearances Effect of age Effect of breed, fever, sex, …. A foal is not only a small horse

16. AMD: protein binding • MIC are free concentrations • Only the free concentration is active • No example of drug/drug interaction leading to increase the free drug concentration by displacement (eg with NSAID) Low or high?

17. AMD: bioavailability Low or high? Large influence of the route of administration and of the formulations

18. Bioavailability • Bioavailability quantifies the proportion of a drug that is absorbed and available to produce its systemic effect • Extent (overall exposure) • Rate (T>MIC)

19. Bioavailability Definition • Absolute • amount of administered drug which enters the systemic (arterial) circulation and the rate at which the drug appears in the blood stream • Relative • to compare formulations (bioequivalence) • to compare routes of administration

20. IV route of administrationby definition F=100% Not always the case for AMD administered as prodrugsuch as esters as erythromycinestolate

21. Oral route of administration

22. Oral route: several possible modalities Mash Intragastric Perlingual Fed vs unfed (foodwithheld for 12h )

23. Oral enrofloxacin : no food effect 5 mg/kg Steinman et al JPT 2006

24. Rifampin administration before and after feeding Bioavailability: 68% (fasted) vs 26% (fed)

25. Influence of food on the F% of erythromycin (base) Food withheld=26% (6-44%) Foalsshouldbe given ERY before they are fed hay. Administration of ERY to foals from which food was withheld overnight apparently provides plasma concentrations of erythromycin A that exceed the minimum inhibitory concentration of Rhodococcusequi for approximately 5 hours. The dosage of 25 mg/kg every 8 hours, PO, appears appropriate. Fed =7.7% (1-18%) Lakritz et al AJVR, Vol 61, No. 9, September 2000

26. Why a possible low oral bioavailability • Poor stability in the stomach • pH effect • Poor absorption • Physiological origin • Binding to cellulosis • Hepatic first-pass effect • Can be predicted from the blood clearance • Drug interaction

27. In vitro binding (%) of TMP and sulphachlorpyridazine to hay, grass silage and concentrate Van Duijkeren, 1996

28. The pH effect(stomach)

29. Poor stability of the AM in the stomach: the case of erythromycin • Inactivated by gastric acid thus: • Enteric-coated formulations • Esters (prodrugs) with improved acid stability but requiring hydrolysis by esterases • Estolate • Stearate • ethyl succinate However a horse and a man can be different and extrapolation misleading

30. Gastric pH Fasted Low pH (average of 1.6) Continuoussecretion Hay ad libitum Bufferingcapacity of hay and saliva (at eachpeak)

31. Erythromycin: bioinequivalence of the different forms • Three possible forms for an oral administration • Erythromycin base • Erythromycin salt (lactobionate, phosphate…) • Erythromycin esters absorbed by the GIT (estolate, etylsuccinate) • Erythromycin ester hydrolysed in the GIT (stearate)

32. Effect of age on bioavailability

33. Age effect: Bioavailability of IG Cefadroxilin foal Duffee JVPT 1997 20 427

34. Effect of age on bioavailability of oral penicillins in the horse

35. Why a possible low oral bioavailability • Poor stability in the stomach • pH effect • Poor absorption • Physiological origin • Binding to cellulosis • Hepatic first-pass effect • Can be predicted from the blood clearance • Drug interaction

36. Poor absorption due to drug-drug interaction

37. Association of AMDClarithromycin ± Rifampin • After RIF comedication, relative bioavailability of CLR decreased by more than 90%. • the drastic lowering of the average CLR plasma concentrations by more than 90% have resulted from induction of hepatic and intestinal CYP3A4 and intestinal ABCB1 and probably • ABCC2. efflux transport seems to be the major reason for lower bioavailability • there are many doubts from a pharmacokinetic point of view that combination therapy of CLR with RIF might really be superior to other eradication protocols as suggested by the results of a retrospective clinical study in foals (Gigue`re et al., 2004). The absence of major drug interactions as shown in our recent pharmacokinetic study with tulathromycinand RIF should be confirmed before a combination treatment is launched in clinical practice (Venner et al., 2010).

38. Poor bioavailability due to a hepatic first-pass effect

39. The 3 segments of the digestive tract in terms of first-pass effect Rectal Limited first-pass effect Buccal cavity No first-pass effect Small intestine/large bowel Full First pass-effect

40. Hepatic first pass effect Eythromycin Dose 30% Liver Fmax = 1 - Eh Eh~70% Fraction eliminated by first pass effect • Fmax = 1 – Eh=1 - [Clh / Qh]=1-[17/24]=0.30

41. Plasma erythromycin after an IG administration of a salt (phosphate) or an ester (estolate) of erythromycin (food withheld) F% from Phosphate:16±3.5% F% fromestolate: 14.7±11% Both are verylow: why? Plasma clearance of erythromycin is very large (17.5ml/kg/min) suggesting a likely large hepatic first-pass effect in horse

42. Intramuscular administration

43. IV administration of sodium benzylpenicillin

44. Penicillin G potassium vs. Penicillin G procaine Flip-flop kinetics Procaine benzylpenicillin( procaine penicillin) is an ester of benzylpenicillin and the local anaesthetic agent procaine. Following deep intramuscular injection, it is slowly absorbed into the circulation and hydrolysed to benzylpenicillin This combination is aimed at reducing the pain and discomfort associated with a large intramuscular injection of penicillin.

45. Influence of the injection site on bioavailability of Penicillin (administration of procaine benzylpenicilin) Semi-membrane / semi-tendineux 4 M. serratus M. biceps M. pectoralis M. gluteus M. Subcutaneous 3 2 Concentrations (UL/mL) 1 (Time) 0 2 4 0 12 10 24h 6 8 Firth et al. 1986, Am. J. Vet. Res.

46. Terminal half-life and bioavailability of procaine benzylpenicillin in the horse The terminal half-life is much more longer after an extravascular administration: The so-called flip-flop phenomenon

47. Intra- vs intermuscular administration • The best site for IM administration is the 5th cervical vertebra, ventral to the funicular part of the ligamentum nuchae but dorsal to the brachiocephalic muscle True IM Boyd et al,1987, Vet. Rec.

48. Intra- vs intermuscular administration • Injection in the 4th space but the ventral injection has traversed to the 6th vertebral space Boyd et al,1987, Vet. Rec.

49. Procaine penicillin adverse effects • PP is associated with incidence of severe adverse reactions with distress…...but much less frequently with water-soluble salts of Penicillin. • Anaphylactic reaction: rare in horses • Penicillin have affinity to proteins and may form hapten • Hypersensitivity is the most common cause of negative reaction to penicillin • Procaine toxicity: frequent in horses • Due to action of the free procaine on the CNS

50. Procaine penicillin adverse effects • Procaine is hydrolysed by plasma esterase to non toxic metabolite (Para-aminobenzoic acid and Diaminoethanol) • Toxicity is observed if the rate of Procaine absorption exceeds the hydrolyzing capacity • Inadvertent IV route after an IM administration • Poor esterase activity (next slide) • Some formulations have high free procaine concentration (vehicule) and this is increase by high room temperature (stability issue)