Ocular penetration of oral doxycycline in the horse

Doxycycline Ameliorates the Severity of Experimental Proliferative Vitreoretinopathy in Mice

After successful surgeries for patients with rhegmatogenous retinal detachment, the most common cause of retinal redetachment is proliferative vitreoretinopathy (PVR), which causes severe vision impairment and even blindness worldwide. Until now, the major treatment for PVR is surgical removal of the epiretinal membrane, while effective treatment to prevent PVR is still unavailable. Therefore, we investigated the potential of doxycycline, an antibiotic in the tetracycline class, to treat PVR using a mouse model. We used the human retinal pigment epithelial cell line, ARPE-19, for in vitro and in vivo studies to test doxycycline for PVR treatment. We found that doxycycline suppressed the migration, proliferation, and contraction of ARPE-19 cells with reduced p38 MAPK activation and total MMP activity. Intravitreal doxycycline and topical tetracycline treatment significantly ameliorated the PVR severity induced by ARPE-19 cells in mice. PVR increased the expression of MMP-9 and IL-4 and p38 MAPK phosphorylation and modestly decreased IL-10. These effects were reversed by doxycycline and tetracycline treatment in the mouse retina. These results suggest that doxycycline will be a potential treatment for PVR in the future.

Nonlinear Mixed-Effect Pharmacokinetic Modeling and Distribution of Doxycycline in Healthy Female Donkeys after Multiple Intragastric Dosing-Preliminary Investigation

Doxycycline (DXC) is a broad-spectrum antibacterial antimicrobial administered to horses for the treatment of bacterial infections which may also affect donkeys. Donkeys have a different metabolism than horses, leading to differences in the pharmacokinetics of drugs compared to horses. This study aimed to describe the population pharmacokinetics of DXC in donkeys. Five doses of DXC hyclate (10 mg/kg) were administered via a nasogastric tube, q12 h, to eight non-fasted, healthy, adult jennies. Serum, urine, synovial fluid and endometrium were collected for 72 h following the first administration. Doxycycline concentration was measured by competitive enzyme immunoassay. Serum concentrations versus time data were fitted simultaneously using the stochastic approximation expectation-maximization algorithm for nonlinear mixed effects. A one-compartment model with linear elimination and first-order absorption after intragastric administration, best described the available pharmacokinetic data. Final parameter estimates indicate that DXC has a high volume of distribution (108 L/kg) as well as high absorption (10.3 h^-1) in donkeys. However, results suggest that oral DXC at 10 mg/kg q12 h in donkeys would not result in a therapeutic concentration in serum, urine, synovial fluid or endometrium by comparison to the minimum inhibitory concentration of common equine pathogens. Further studies are recommended to identify appropriate dosage and dosing intervals of oral DXC in donkeys.

Pharmacokinetics and pharmacodynamics of doxycycline in a Streptococcusequi subsp. zooepidemicus infection model in horses

The objectives of this study were to investigate the pharmacokinetics (PK), pharmacodynamics (PD), and the efficacy of oral administration of doxycycline (DXC) in horses with Streptococcus zooepidemicus tissue infections. Tissue chambers (TC) were implanted subcutaneously in the cervical region of 7 horses and inoculated with a single S. zooepidemicus isolate with a minimum inhibitory concentration (MIC) of 0.25 µg/ml, determined by agar dilution. Doxycycline hyclate (10 mg/kg, orally, q 12 h, for 5 days) mixed with poloxamer gel was started following inoculation. The TC fluid was sampled prior to and following inoculation for cytology analysis, quantitative culture, and DXC determination. Plasma DXC concentrations were measured over 48 h following the last dose of DXC administered. The mean plasma peak concentration (C_max ) of DXC was 0.32 µg/ml, and concentrations above the MIC were only reached in 3 TC samples. In plasma, mean T > MIC was 2.4 h, mean C_max /MIC was 1.30, and mean AUC_last /MIC was 11.63 h. These PK/PD indices did not reach the suggested targets for DXC treatments of infections, and the TC abscessed in all horses. This is the first study to evaluate the recommended dose of DXC in horse in an infection model.

Leptospirosis: An important infectious disease in North American horses

North American horses are commonly exposed to Leptospira organisms. Leptospira Bratislava is the most common infecting serovar but this serovar has not been confirmed to cause clinical disease in North American horses. Leptospira Pomona type kennewicki is responsible for most of the clinical diseases (leptospirosis) in North American horses. Leptospirosis is most commonly associated with diseases of the placenta and fetus, the kidneys and the eyes in horses. In-utero infections in pregnant mares may result in abortion, neonatal illness or birth of an antibody positive healthy foal. Acute renal failure in younger horses and recurrent uveitis in adult horses are other well documented clinical syndromes of leptospirosis. Abortions, neonatal disease and acute renal failure are caused by a subacute infection, while horses with Leptospira associated recurrent uveitis develop ocular disease months or years after the initial Leptospira infection. Diagnosis of Leptospirosis is made by a combination of antigen or antibody testing methods. Mares that abort following Leptospira infection have no additional clinical signs at the time of abortion but may shed the offending Leptospira spp. in the urine for several weeks. Antibiotic treatments are sometimes used in hopes of decreasing Leptospira shedding in infected horses or prophylactically in exposed pregnant mares but documentation of efficacy is lacking. Horses with Leptospira - associated acute renal failure can be successfully treated with antibiotics and supportive care. Recurrent uveitis is commonly associated with leptospirosis in North American horses and although horses may have chronic intraocular infection triggering an immune disease, systemic antimicrobial therapy has not been effective in eliminating the organism from the eye. An equine approved Leptospira Pomona type kennewicki vaccine is now available in North America.

In vitro susceptibility of Borrelia burgdorferi isolates to three antibiotics commonly used for treating equine Lyme disease

Background

Lyme disease in humans is predominantly treated with tetracycline, macrolides or beta lactam antibiotics that have low minimum inhibitory concentrations (MIC) against Borrelia burgdorferi. Horses with Lyme disease may require long-term treatment making frequent intravenous or intramuscular treatment difficult and when administered orally those drugs may have either a high incidence of side effects or have poor bioavailability. The aim of the present study was to determine the in vitro susceptibility of three B. burgdorferi isolates to three antibiotics of different classes that are commonly used in practice for treating Borrelia infections in horses.

Results

Broth microdilution assays were used to determine minimum inhibitory concentration of three antibiotics (ceftiofur sodium, minocycline and metronidazole), for three Borrelia burgdorferi isolates. Barbour-Stoner-Kelly (BSK K + R) medium with a final inoculum of 10⁶Borrelia cells/mL and incubation periods of 72 h were used in the determination of MICs. Observed MICs indicated that all isolates had similar susceptibility to each drug but susceptibility to the tested antimicrobial agents varied; ceftiofur sodium (MIC = 0.08 μg/ml), minocycline hydrochloride (MIC = 0.8 μg/ml) and metronidazole (MIC = 50 μg/ml).

Conclusions

The MIC against B. burgorferi varied among the three antibiotics with ceftiofur having the lowest MIC and metronidazole the highest MIC. The MIC values observed for ceftiofur in the study fall within the range of reported serum and tissue concentrations for the drug metabolite following ceftiofur sodium administration as crystalline-free acid. Minocycline and metronidazole treatments, as currently used in equine practice, could fall short of attaining MIC concentrations for B. burgdorferi.

Prognosis and impact of equine recurrent uveitis

REASONS FOR PERFORMING STUDY

Equine recurrent uveitis (ERU) is a leading cause of vision loss in horses.

OBJECTIVES

To assess the prognosis and impact of ERU on affected horses and their owners by evaluating the signalment, treatment and outcome (including the loss of use, vision assessment and economic loss).

STUDY DESIGN

Retrospective impact study.

METHODS

Medical records of horses presenting to the North Carolina State University Veterinary Health Complex (NCSU-VHC) with ERU between 1999 and 2014 were reviewed. Signalment, clinical signs, ophthalmic examination findings, treatments and outcomes were evaluated. Owner questionnaires were completed regarding vision, job/role, monetary value, diagnostic and treatment costs, concurrent illness and outcomes.

RESULTS

Records of 224 horses (338 eyes) with ERU were reviewed. There was an overrepresentation of Appaloosas (54; 24.1%), Hanoverians (11; 4.9%) and other Warmbloods (13; 5.8%). Ninety-six eyes (28.4%) were diagnosed as blind and 38 eyes (11.2%) with glaucoma on initial evaluation. Leptospirosis titres of serum and/or aqueous humour were obtained in 88 horses and were positive in 40 horses (45.5%), with L. pomona being the most frequently isolated serovar. Globe loss at the NCSU-VHC occurred in 41 ERU eyes (12.1% of total). Owner questionnaires were evaluated in 194 horses (86.6%) and 91 horses (46.9%) were reported blind in the affected eye(s). Fifty-seven horses (29.4%) did not return to their previous role, while 61 (31.4%) performed at a reduced level. Equine recurrent uveitis decreased the monetary value of 164 horses. Twenty-nine horses (14.9%) were euthanised and 37 (19.1%) underwent change in ownership as a direct result of ERU.

CONCLUSIONS

The impact of ERU is attributed to the high frequency of blindness, globe loss and loss of function. Euthanasia and change of ownership are common sequelae to the progressive nature of ERU. These factors, along with financial costs of the disease, have a significant impact on affected horses and their owners.

Plasma matrix metalloproteinase activity in horses after intravenous infusion of lipopolysaccharide and treatment with matrix metalloproteinase inhibitors

OBJECTIVE

To establish an in vivo method for matrix metalloproteinase (MMP)-2 and MMP-9 induction in horses via IV administration of lipopolysaccharide (LPS) and to evaluate the ability of doxycycline, oxytetracycline, flunixin meglumine, and pentoxifylline to inhibit equine MMP-2 and MMP-9 production.

ANIMALS

29 adult horses of various ages and breeds and either sex.

PROCEDURES

In part 1, horses received an IV administration of LPS (n = 5) or saline (0.9% NaCl) solution (5). Venous blood samples were collected before and at specified times for 24 hours after infusion. Plasma was harvested and analyzed for MMP-2 and MMP-9 activities via zymography. In part 2, horses received doxycycline (n = 5), oxytetracycline (5), flunixin meglumine (5), or pentoxifylline (4) before and for up to 12 hours after administration of LPS. Plasma was obtained and analyzed, and results were compared with results from the LPS-infused horses of part 1.

RESULTS

Administration of LPS significantly increased MMP-2 and MMP-9 activities in the venous circulation of horses. All MMP inhibitors significantly decreased LPS-induced increases in MMP activities but to differing degrees. Pentoxifylline and oxytetracycline appeared to be the most effective MMP-2 and MMP-9 inhibitors, whereas doxycycline and flunixin meglumine were more effective at inhibiting MMP-2 activity than MMP-9 activity.

CONCLUSIONS AND CLINICAL RELEVANCE

IV administration of LPS to horses caused increased venous plasma activities of MMP-2 and MMP-9. These MMP activities were reduced by pentoxifylline and oxytetracycline, suggesting that further evaluation of these medications for treatment and prevention of MMP-associated diseases in horses is indicated.

Oral doxycycline reduces pterygium lesions; results from a double blind, randomized, placebo controlled clinical trial

PURPOSE

To determine whether oral doxycycline treatment reduces pterygium lesions.

DESIGN

Double blind, randomized, placebo controlled clinical trial.

PARTICIPANTS

98 adult patients with primary pterygium.

METHODS

Patients were randomly assigned to receive 100 mg oral doxycycline twice a day (49 subjects), or placebo (49 subjects), for 30 days. Photographs of the lesion were taken at the time of recruitment and at the end of the treatment. Follow-up sessions were performed 6 and 12 months post-treatment. Statistical analyses for both continuous and categorical variables were applied. p values of less than 0.05 were considered to indicate statistical significance.

MAIN OUTCOME MEASURES

The primary endpoint was the change in lesion size after 30 days of treatment.

RESULTS

The primary endpoint was not met for the whole population but subgroup analysis showed that doxycycline was effective in patients of Caucasian origin while other ethnicities, mostly Hispanic, did not respond to the treatment. Moreover, there was a correlation between age and better response (p = 0.003). Adverse events were uncommon, mild, and in agreement with previous reports on short doxycycline treatments.

CONCLUSIONS

Oral doxycycline was superior to placebo for the treatment of primary pterygia in older Caucasian patients. These findings support the use of doxycycline for pterygium treatment in particular populations.

TRIAL REGISTRATION

European Union Clinical Trials Register EudraCT 2008-007178-39.

Orally administered doxycycline accumulates in synovial fluid compared to plasma

REASONS FOR PERFORMING STUDY

Tetracycline compounds have been used to slow the progression of osteoarthritis (OA) and rheumatoid arthritis but the concentration of doxycycline attained in synovial fluid following oral, low-dose administration has yet to be determined.

OBJECTIVE

To determine the concentration of doxycycline in synovial fluid following oral, low-dose administration.

METHODS

Six mature horses received doxycycline (5 mg/kg bwt q. 12 h for 5 doses). Venous blood and synovial fluid samples were collected at t=0, 0.25, 0.5, 1, 12, 24, 48 and 72 h. Doxycycline concentrations were measured using reverse phase high pressure liquid chromatography with ultraviolet detection.

RESULTS

Doxycycline concentrations at all time points after t=0 were above the lower limit of quantification for the assay. Plasma concentrations of doxycycline were above 0.21 microg/ml at t=0.5 h. The mean+/-s.d. peak concentration (Cmax) of doxycycline in plasma was 0.37+/-0.22 microg/ml and time to peak concentration was 0.54+/-0.19 h. Synovial fluid concentrations of doxycycline were above 0.12 microg/ml 1 h after drug administration. The mean Cmax of doxycycline in the synovial fluid was 0.27+/-0.10 microg/ml. The penetration factor of doxycycline from plasma into synovial fluid, as determined by a ratio of the area-under-the-curve for synovial fluid:plasma during the sampling period, was 4.6.

POTENTIAL RELEVANCE

Orally administered doxycycline distributes easily into synovial fluid with a penetration factor of 4.6. Terminal half-life of the drug in synovial fluid was longer than in the plasma, indicating possible accumulation in this compartment. Further in vivo studies are warranted to define a medication protocol prior to routine clinical use of doxycycline for the treatment of OA.

Doxycycline levels in preocular tear film of horses following oral administration

OBJECTIVE

To determine the concentration of doxycycline in preocular tear film following oral administration in horses as a possible therapeutic modality for infectious and keratomalacic equine keratitis.

PROCEDURE

Eight broodmares without ocular disease from a Thoroughbred breeding facility were included in this study. Each mare received 20 mg/kg of doxycycline by mouth once daily in the morning for five consecutive days. Tears were collected 1 h after doxycycline administration starting on day one of administration and continuing for 10 consecutive days. Doxycycline levels in the tears were measured using liquid chromatography with tandem mass spectrometric detection (LC-MS/MS).

RESULTS

Doxycycline was present in the tears of each mare at low microg/mL levels with the highest concentration appearing on the third to fifth days (8.21-9.83 microg/mL). Doxycycline levels had fallen below quantifiable ranges by day 10. No systemic side-effects were noted in any of the horses included in this study.

CONCLUSIONS

Oral doxycycline is present in preocular tear film of normal horses with noninflamed eyes and may be useful as treatment in equine ulcerative keratomalacia. The oral dose listed was tolerated well by the horses in this study. The drug levels attained at 20 mg/kg once daily orally of doxycycline may aid in the treatment of corneal ulceration in horses, but further study is warranted.

Ocular penetration of intravenously administered enrofloxacin in the horse

REASON FOR PERFORMING STUDY

Information on antibiotic concentrations in the equine eye following systemic therapy is limited. Reports that Leptospira spp. are frequently present in the eyes of horses with recurrent uveitis, emphasises a need for studies on ocular concentrations of specific antibiotics.

HYPOTHESES

1) Enrofloxacin, administered i.v. at 7.5 mg/kg bwt q. 24 h, results in aqueous humour concentrations greater than the reported minimum inhibitory concentration (MIC) for Leptospira pomona. 2) Aqueous humour paracentesis sufficiently disrupts the blood-aqueous humour barrier (BAB) to cause an increase in aqueous humour protein and enrofloxacin concentrations.

METHODS

Aqueous humour enrofloxacin and total protein concentrations were determined in 6 healthy, mature horses after i.v. administration of enrofloxacin. Paracentesis was performed on the left eye on Days 3 and 4, 1 h following enrofloxacin administration, to determine enrofloxacin concentrations in healthy eyes and in eyes with mechanical disruption of the BAB. Paracentesis was also performed on the right eye 23 h after enrofloxacin administration. Blood samples were collected from the horses at identical times to determine enrofloxacin aqueous humour:plasma ratios.

RESULTS

Mean +/- s.d. enrofloxacin concentration in the aqueous humour 1 h post administration on Day 3 was 0.32 +/- 0.10 mg/l (range 0.18-0.47); and aqueous humour enrofloxacin, total protein and aqueous humour:plasma enrofloxacin ratios were higher on Day 4 than Day 3.

CONCLUSIONS AND POTENTIAL RELEVANCE

Following disruption of the BAB, enrofloxacin concentrations were above the reported MIC for Leptospira pomona.

Pharmacokinetics of oral doxycycline and concentrations in body fluids and bronchoalveolar cells of foals

The objective of this study was to determine the disposition of orally administered doxycycline in foals. Six healthy 4- to 8-week-old foals were used. Doxycycline was administered to each foal via the intragastric (IG) route at dosages of 10 and 20 mg/kg, in a cross-over design. After the first 10 mg/kg dose, five additional doses were administered at 12-h intervals. A microbiological assay was used to measure doxycycline activity in serum, urine, peritoneal fluid, synovial fluid, cerebrospinal (CSF), pulmonary epithelial lining fluid (PELF), and bronchoalveolar (BAL) cells. Following administration at 10 mg/kg, mean+/-SD time to peak serum doxycycline activity (tmax) was 3.0+/-1.2 h, maximum serum activity (Cmax) was 2.54+/-0.27 microg/mL, and terminal half-life (t1/2) was 8.5+/-2.8 h. Administration at a dose of 20 mg/kg resulted in a significantly longer tmax (5.5+/-1.8 h) as well as a tendency toward higher Cmax (2.89+/-0.33 microg/mL) and longer t1/2 (11.9+/-2.6 h). After multiple IG doses, doxycycline activity in CSF was significantly lower than concurrent serum activity, whereas peritoneal fluid, synovial fluid, and BAL cell doxycycline activity was similar to concurrent serum activity. Doxycycline activity in urine and PELF was significantly higher than that found at other sites. Oral administration at a dosage of 10 mg/kg every 12 h would maintain serum, PELF, and BAL cell activity above the minimum inhibitory concentrations of Rhodococcus equi, beta-hemolytic streptococci, and other susceptible bacterial pathogens for the entire dosing interval.

Pharmacokinetics and tissue distribution of doxycycline after oral administration of single and multiple doses in horses

OBJECTIVE

To determine pharmacokinetics, safety, and penetration into interstitial fluid (ISF), polymorphonuclear leukocytes (PMNLs), and aqueous humor of doxycycline after oral administration of single and multiple doses in horses.

ANIMALS

6 adult horses.

PROCEDURE

The effect of feeding on drug absorption was determined. Plasma samples were obtained after administration of single or multiple doses of doxycycline (20 mg/kg) via nasogastric tube. Additionally, ISF, PMNLs, and aqueous humor samples were obtained after the final administration. Horses were monitored for adverse reactions.

RESULTS

Feeding decreased drug absorption. After multiple doses, mean +/- SD time to maximum concentration was 1.63 +/- 1.36 hours, maximum concentration was 1.74 +/- 0.3 microg/mL, and elimination half-life was 12.07 +/- 3.17 hours. Plasma protein binding was 81.76 +/- 2.43%. The ISF concentrations correlated with the calculated percentage of non-protein-bound drug. Maximum concentration was 17.27 +/- 8.98 times as great in PMNLs, compared with plasma. Drug was detected in aqueous humor at 7.5% to 10% of plasma concentrations. One horse developed signs of acute colitis and required euthanasia.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that doxycycline administered at a dosage of 20 mg/kg, PO, every 24 hours will result in drug concentrations adequate for killing intracellular bacteria and bacteria with minimum inhibitory concentration < or = 0.25 microg/mL. For bacteria with minimum inhibitory concentration of 0.5 to 1.0 microg/mL, a dosage of 20 mg/kg, PO, every 12 hours may be required; extreme caution should be exercised with the higher dosage until more safety data are available.