**2.2 Intramammary antimicrobial treatments (IMM)**

The IMM infusion is the more used administration route in the ATM treatment of mastitis. However, many IMM products have been released to the market without the necessary scientific support about its PK behavior and studies about its clinical and bacteriological efficacy. The benefits of IMM administration are the high concentrations reached in milk and less loss due to drug absorption and transfer processes through biological membranes. While the disadvantages of this route may be the uneven distribution of various compounds within the udder, the risk of mammary contamination by bacteria inoculation through the teat canal and the possible irritation of tissue breast by the formulation (Gruet et al., 2001). Even in vitro studies have shown that ATMs administered by IMM route can negatively affect the phagocytic process in the mammary environment (Nickerson et al., 1985; 1986).

After administration of an IMM infusion, the contact between the ATM agent and the pathogen within the mammary gland is subject to a series of successive events (Ziv, 1980c; Mestorino, 1993a):


428 A Bird's-Eye View of Veterinary Medicine

That is to say that the majority of the drugs can be ionized or nonionized according to its pKa and the pH of the surrounding environment. Nonionized compounds have a higher lipid-water partition coefficient than the ionized ones and thus it is easier for them to diffuse through lipidic membranes. The amphoteric molecules, such as danofloxacin (pKa = 6.2 to 9.4) does not depend on the relationship pK/pH and therefore its distribution is essentially determined by the degree of lipid solubility of the molecule and consequently by its lipid-

Table 2 shows different ATMs with its theoretical and experimental milk: plasma ratios. Observing this table we can conclude that the diffusion of organic acids into milk is highly predictable, but the diffusion of organic bases can be predicted only when these are largely

**Sulfanilamide** Acid 10.4 Moderate 1.00 – 0.97 **Sulfathiazole** Acid 7.1 Moderate 0.37 – 0.35 **Sulfadiazine** Acid 6.5 Moderate 0.28 – 0.21 **Penicillin G** Acid 2.8 Moderate 0.16 – 0.20 **Cloxacillin** Acid 2.8 High 0.16 – 0.22

**Cephacetrile** Acid 2.4 Moderate 0.12 – 0.15 **Cephapirin** Acid 2.6 Moderate 0.14 – 0.18

**Rifampicin** Acid 7.9 High 0.85 – 1.10 **Novobiocin** Acid 4.3 High 0.30 – 0.,33 **Penethamate** Base 8.5 High 5.7 – 6.1

**neomycin** Base 8.9 Low 7.5 – 0.5

**clindamycin** Base 7.6 High 4.2 – 4.4

**Tetracyclines** Amphoteric - Moderate 0.4; 0.8 – 0.6; 1.4 Table 2. Partition of ATMs in plasma and milk in lactating animals. From Ziv G. (1980b)

**Polymyxin B, colistin** Base 10.0 Very low 8.0 – 0.3 **Erythromycin** Base 8.8 High 6.5 – 8.5 **Tylosin** Base 7.1 High 5.0 – 4.5 **Spiramycin** Base 8.2 High 4.8 – 4.6

**Lipid solubility** 

7.2 High 0.26 – 0.26

7.2 High 0.26 – 0.26

6.7 Moderate 0.25 – 0.25

**Milk to serum concentration ratio Theoretical - Experimental** 

nonionized in plasma and have a moderate degree of lipid solubility.

**nature PKa**

water partition coefficient.

**Antimicrobial agent Chemical** 

**Ampicillin** Acid 2.8;

**Amoxicillin** Acid 2.8;

**Rifamycin SV** Acid 2.8;

**Streptomycin,** 

**Lincomycin,** 


Pharmacokinetic-Pharmacodynamic Considerations for Bovine Mastitis Treatment 431

The milk: plasma absorption phenomenon results in the presence of the ATM agent in blood plasma. Once in the systemic circulation, the drug will be object of the PKs phenomena

The ATM distribution inside the gland occurs by passive diffusion of the molecules through the lipophilic and hydrophilic components of the glandular secretion. This phenomenon is affected by drug binding to breast tissue and / or components of milk (Mestorino, 1993a). Once the drug establishes contact with the glandular cells, its penetration into tissue continues depending on lipophilicity. As will be detailed later, cell penetration is a critical point in certain infections. The excretion process of the ATM from the mammary gland is governed by the kind of excipient used, the quantity of milk produced, the molecule characteristics, the health of the mammary gland, the number of daily milkings made (Ziv,

After the IMM treatment at cow drying, the ATMs elimination from the mammary gland can be expressed by a monoexponential or biexponential curve. The elimination rate is affected by the dose, the nature of the vehicle, the drug characteristics, and the extent of binding of the antibacterial agent to the gland content and to the mammary tissue (Ziv,

Successful ATM chemotherapy depends on a correct diagnosis, selection of the appropriate

When considering the choice of ATM agent and dosage regimen, we need to consider the PKs of the chosen drug in the target animal species and the PD indices that drive its clinical effectiveness. For example, penicillins, like all β-lactam ATMs (penicillins, cephalosporins, carbapenems and monobactams), exhibit time-dependent killing. This means that maximum clinical effectiveness is achieved by ensuring that the free serum concentration of the selected β-lactam exceeds the MIC of the pathogen for the appropriate percentage of the dosing interval. If the pathogen is a Gram-positive organism, the targeted duration is usually ≥40% of the dosing interval. Instead, concentrations of most β-lactams should exceed the MIC of the pathogen by ≥80% of the dosing interval when the infectious agent is a Gram-negative organism. In other words, for drugs exhibiting time-dependent killing, increasing the concentration of the drug in excess of the MIC of the pathogen does not increase the killing rate. Rather, the extent of killing is dictated by the duration of time that

On the other hand, other bactericidal ATM agents, as fluoroquinolones and aminoglycosides, exhibit concentration-dependent killing. In this situation, the rate of killing increases as the drug concentration increases above the MIC of the bacterial pathogen. Thus, ATM agents may be classified as those that exhibit time-dependent killing with null or brief post-antibiotic effect (e.g. β-lactams), time-dependent killing with prolonged post-antibiotic effect (e.g. glycopeptides), concentration dependent killing (e.g. fluorquinolones and aminoglycosides), and those that are generally considered to be

bacteriostatic (e.g. tetracyclines, macrolides, lincosamides and phenicols).

taking place after absorption (distribution, metabolism and excretion) (Ziv, 1980b).

1980c), the dose and total volume of formulation among other factors.

ATM agent, and its administration with an adequate dosing scheme.

1980c; Mestorino, 1993a; Mattie et al., 1997).

bacteria are exposed to the drug.

**3. Antimicrobial pharmacodynamic concepts** 

The kind and proportions of the pharmaceutical excipients contained in the formulation are which determines the rate of drug release. They define the pharmaceutic phase which governs the initial shape of the ATM milk concentration versus time curve.

IMM formulations used for mastitis treatment during lactation contain pharmaceutical excipients that favors quick release of the active components, because the objective is to eradicate the infection of glandular tissue and minimize the withdrawal time (Ziv, 1980c).

On the other hand, when the IMM treatment is during the dry period, the process of release of the active component must be slow and necessarily has great influence on the initial shape of the concentration versus time curve. The excipients used, although very variable, may include vegetable or mineral oils. The mineral oil, such as paraffin or glycerine oils, has the advantage of delaying the release of the active compound and prolonge the permanence of the ATM inside the gland (Ziv, 1980c; Mattie et al., 1997).

In some cases the pharmaceutical excipients contain other agents that increase the retention, such as aluminum monostearate in an oily base, hydroxystearin (Ziv, 1980c). Mercer et al. (1974) studied the absorption in blood and the persistence in milk of penicillin G, after IMM infusion of 400000 IU in mastitic quarters in a rapid (aqueous formulation) and a slow release base (3.6% aluminum monostearate in peanut oil). Penicillin G administered in the slow release formulation could still be detected in the milk 120 h after infusion compared to only 56 h for the aqueous base formulation. These figures must be considered in relation to the MIC of penicillin G for *S. aureus* in milk which is around 0.1 IU.mL-1; this means that active levels were maintained above the MIC for 72 h for the oil-based formulation compared to 32 h for the aqueous-based formulation. After the administration of IMM syringes, part of the formulation will be lost with the milk in the first milking posttreatment. This loss is negligible in the case of treatment at the start of the dry period, because the cows are removed from the milking routine. However, the disruptions of the milking routine trigger physiological changes in the mammary gland that may affect the pharmaceutical and PK phases.

The start of the PK phase presupposes the existence of available drug in the milk secretion and depends on the disintegration of the formulation and the dissolution of the drug. These processes determine the drug availability.

The amount of drug recovered in milk is influenced by several factors, among which we can mention the kind of excipient used in the formulation, the milk: plasma passage rate, the size of the udder and the volume of milk contained in the gland (Ziv, 1980c).

The passage of the antibiotic from milk to serum involves the movement of drug across a biological membrane. The factors influencing this passage are the same that affect their transfer from the blood into the milk: the molecule pKa, the lipid solubility of the nonionized fraction, the percentage of udder tissue binding and the binding to milk proteins (Mestorino, 1993a). Hydro soluble drugs pass through the membrane mainly through the protein channels. Lipid soluble drugs cross the membrane through the lipoproteic region .

The ability to cross the milk: plasma barrier is expressed as lipid solubility coefficient for nonionized fraction percentage (Mattie et al., 1997) and the real rate of passage can be quantified by the Cmax(plasma)/Cmax (milk) or by the AUC(plasma) / AUC(milk) ratios (Ziv, 1980a).

The kind and proportions of the pharmaceutical excipients contained in the formulation are which determines the rate of drug release. They define the pharmaceutic phase which

IMM formulations used for mastitis treatment during lactation contain pharmaceutical excipients that favors quick release of the active components, because the objective is to eradicate the infection of glandular tissue and minimize the withdrawal time (Ziv, 1980c).

On the other hand, when the IMM treatment is during the dry period, the process of release of the active component must be slow and necessarily has great influence on the initial shape of the concentration versus time curve. The excipients used, although very variable, may include vegetable or mineral oils. The mineral oil, such as paraffin or glycerine oils, has the advantage of delaying the release of the active compound and prolonge the permanence

In some cases the pharmaceutical excipients contain other agents that increase the retention, such as aluminum monostearate in an oily base, hydroxystearin (Ziv, 1980c). Mercer et al. (1974) studied the absorption in blood and the persistence in milk of penicillin G, after IMM infusion of 400000 IU in mastitic quarters in a rapid (aqueous formulation) and a slow release base (3.6% aluminum monostearate in peanut oil). Penicillin G administered in the slow release formulation could still be detected in the milk 120 h after infusion compared to only 56 h for the aqueous base formulation. These figures must be considered in relation to the MIC of penicillin G for *S. aureus* in milk which is around 0.1 IU.mL-1; this means that active levels were maintained above the MIC for 72 h for the oil-based formulation compared to 32 h for the aqueous-based formulation. After the administration of IMM syringes, part of the formulation will be lost with the milk in the first milking posttreatment. This loss is negligible in the case of treatment at the start of the dry period, because the cows are removed from the milking routine. However, the disruptions of the milking routine trigger physiological changes in the mammary gland that may affect the

The start of the PK phase presupposes the existence of available drug in the milk secretion and depends on the disintegration of the formulation and the dissolution of the drug. These

The amount of drug recovered in milk is influenced by several factors, among which we can mention the kind of excipient used in the formulation, the milk: plasma passage rate, the

The passage of the antibiotic from milk to serum involves the movement of drug across a biological membrane. The factors influencing this passage are the same that affect their transfer from the blood into the milk: the molecule pKa, the lipid solubility of the nonionized fraction, the percentage of udder tissue binding and the binding to milk proteins (Mestorino, 1993a). Hydro soluble drugs pass through the membrane mainly through the protein channels. Lipid soluble drugs cross the membrane through the lipoproteic region . The ability to cross the milk: plasma barrier is expressed as lipid solubility coefficient for nonionized fraction percentage (Mattie et al., 1997) and the real rate of passage can be quantified by the Cmax(plasma)/Cmax (milk) or by the AUC(plasma) / AUC(milk) ratios (Ziv, 1980a).

size of the udder and the volume of milk contained in the gland (Ziv, 1980c).

governs the initial shape of the ATM milk concentration versus time curve.

of the ATM inside the gland (Ziv, 1980c; Mattie et al., 1997).

pharmaceutical and PK phases.

processes determine the drug availability.

The milk: plasma absorption phenomenon results in the presence of the ATM agent in blood plasma. Once in the systemic circulation, the drug will be object of the PKs phenomena taking place after absorption (distribution, metabolism and excretion) (Ziv, 1980b).

The ATM distribution inside the gland occurs by passive diffusion of the molecules through the lipophilic and hydrophilic components of the glandular secretion. This phenomenon is affected by drug binding to breast tissue and / or components of milk (Mestorino, 1993a). Once the drug establishes contact with the glandular cells, its penetration into tissue continues depending on lipophilicity. As will be detailed later, cell penetration is a critical point in certain infections. The excretion process of the ATM from the mammary gland is governed by the kind of excipient used, the quantity of milk produced, the molecule characteristics, the health of the mammary gland, the number of daily milkings made (Ziv, 1980c), the dose and total volume of formulation among other factors.

After the IMM treatment at cow drying, the ATMs elimination from the mammary gland can be expressed by a monoexponential or biexponential curve. The elimination rate is affected by the dose, the nature of the vehicle, the drug characteristics, and the extent of binding of the antibacterial agent to the gland content and to the mammary tissue (Ziv, 1980c; Mestorino, 1993a; Mattie et al., 1997).
