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CVM GFI #38 Guideline for Effectiveness Evaluation of Topical/Otic Animal Drugs August 1984

Docket Number:
Issued by:
Guidance Issuing Office
Center for Veterinary Medicine

Revised August 21, 1984



I. Introduction
II. Rationale for Proposed Product
III. Definitions of Topical Categories
IV. Twelve Variables Affecting Efficacy
V. Drug Release from Topical/Otic Preparations
VI. Discussion of Animal Models for Efficacy Testing
VII. Classification, Testing Methodology, and Uses of Animal Models
VIII. Inter-Species Extrapolation and Dose Determination
IX. Testing Procedures for Combination Topical/Otic Preparations
A. Non-target Species Studies
B. Target Species Studies
X. Summary


This guideline is intended to aid sponsors of New Animal Drug Applications (NADAs) develop protocols for tests generating data to be included in NADAs to show the effectiveness of topical/otic new animal drugs, and establishes criteria for the evaluation of such data. The term "topical/otic drug" as used in this guideline means an animal drug for topical application, including administration to the external auditory canal, that is intended for the prevention or treatment of a condition affecting, or that is intended to affect the structure or function of, the integument and/or external auditory canal.

This guideline is applicable only to topical/otic products containing any one or a combination of antibacterial, antiinflammatory, and antifungal agents. Topical products containing other types of active ingredients are infrequently presented and are excluded from the scope of this guideline. The principles enunciated in this guideline, and in particular those concerning dose determination and the demonstration of the contribution of each active ingredient, are uniquely designed for, and specifically limited in applicability to, the development and evaluation of the topical/otic drugs within its scope.

This guideline is designed to provide general directions for acquiring information pertinent to effectiveness and to promote uniform review of submissions by the Center for Veterinary Medicine (CVM). Sponsors are invited to confer with appropriate CVM personnel prior to undertaking their studies. Conferences may include total program and individual protocol reviews. Appropriate protocols can be developed by mutual discussion among the sponsor's and CVM's personnel.

Critique of this guideline is encouraged and welcomed. As other methods for the evaluation of topical/otic drugs and/or better experimental designs become available, they may be incorporated as revisions.

In accordance with the agency's regulation at 21 CFR 10.90(b), which states agency policy regarding guidelines, this guideline does not prevent a sponsor from attempting to satisfy the requirements for establishing the effectiveness of topical/otic new animal drugs within its scope by collecting data in manners other than those discussed in the guideline. It is recommended, however, that prior to choosing to follow other procedures, the sponsor's personnel discuss the matter with CVM's personnel to prevent the expenditure of resources on activity that may later be determined to be unacceptable.

II. Rationale for Proposed Product

Initially, the drug sponsor should address the issue of whether there is scientific justification or medical rationale for the proposed topical/otic product. What is the etiology of the clinical condition(s) for which the proposed product is indicated? Is there an identifiable population of animals that would benefit from the use of the proposed product containing one or more active ingredients (antiinflammatory, antibacterial, antifungal)? For example, attached Exhibit A provides an etiological and epidemiological exposition of canine and feline otitis externa. The same methods/principles utilized in Exhibit A are applicable for documenting topical/otic product rationale for other treatable conditions and different species. Complete citations to all references relied upon should be included when documenting the rationale for a new product.

III. Definitions of Topical Categories

The classes of topical/otic products discussed in this guideline exert proposed beneficial effect(s) by either the treatment or prophylaxis of skin infections and or inflammatory processes.

  1. First-aid product. A safe, non-irritating preparation to be applied to adequately cleansed small superficial wounds, such as cuts, abrasions and burns, that reduces microbial numbers and/or aids in prevention of infection and does not interfere with normal wound healing.
  2. Skin wound protectant. A safe, non-irritating preparation to be applied to adequately cleansed small wounds that provides a protective physical barrier, and neither delays healing nor favors the growth of microorganisms. Reference: 39 FR 33140 (September 13, 1974) regarding barrier testing for skin wound protectants.
  3. Skin wound antibiotic/antifungal. A safe, non-irritating antibiotic and/or antifungal preparation that prevents or treats overt skin infection. Claims stating or implying an effect against microorganisms must be supported by controlled studies in the target species that demonstrate effectiveness in the prevention or treatment of infection.
  4. Skin wound antiinflammatory agent: A safe, naturally occurring or synthesized steroidal or other agent used to reduce inflammation and thereby inhibit pruritis associated with surface cuts, abrasions, burns or steroid responsive dermatoses.

IV. Twelve Variables Affecting Efficacy

The following are 12 variable factors that significantly affect the efficacy determination of topical/otic preparations:

  1. Amount of drug interfacing with skin
  2. Concentration/dilution of active ingredients
  3. Type of vehicle
  4. Solubility in vehicle
  5. Rate of release, penetration, absorption, etc.
  6. Surface area medicated
  7. Frequency and manner of application
  8. Contact time
  9. Degree of excoriation of lesion
  10. Accuracy of diagnosis (including characteristics of etiological agent(s), pathogenicity, resistance, etc.)
  11. Quantity of hair
  12. Presence of organic matter

V. Drug Release from Topical/Otic Preparations

It is essential to consider the influence of the vehicle on the effectiveness of a topical preparation. The vehicle may be the controlling factor in the release of the active ingredient(s) from the preparation. The following discussion serves to clarify some of the problems involved in trying to assess drug release and the resulting effectiveness.

Drug release from any dosage form occurs at the interface between the vehicle and the tissue to which it is applied. The drug must leave the vehicle and enter the environment of the tissue before it can exert any biological activity. There are many factors that influence the rate and extent of this drug release. Some of these factors are: concentration of the drug in the vehicle; solubility of the drug in the vehicle; diffusion coefficient of the drug in the vehicle; partition coefficient of the drug between the vehicle and the tissue. Influencing each of these factors are other conditions such as pH of the vehicle, drug or tissue; temperature; degree of hydration of skin, etc. Obviously, drug release from dosage form is not merely a simple matter of smearing on a product and obtaining instantaneous results. Many physico-chemical interactions eventually determine success or failure of a product, apart from any activity of the active ingredient(s).

From a physico-chemical viewpoint, an insoluble particle in a fairly viscous, greasy vehicle would not be an ideal dosage form. If little or no diffusion through the vehicle takes place, then the only portion of active ingredient available for biologic activity is that which exists at the tissue interface at the time of application or that which later comes in contact with the receding boundary of the ointment. Perhaps it is for this reason that demonstration of effectiveness of topical/otic ointments has been difficult in some well-controlled studies.

Ointments have previously been the most widely used topical/otic drug dosage forms. An occlusive vehicle (one that acts as a physical barrier), such as an ointment, has certain advantages in treating small wounds, e.g., may aid in wound healing. However, a higher concentration of the active ingredient can be made available to the tissue site over a short period of time from a cream formulation than from an ointment formulation. Other topical/otic dosage forms (gels, solutions, lotions, etc.) have unique advantages and disadvantages.

The release of drugs from various vehicles has been the subject of many reviews and research papers. Consistently, these papers indicate that a formulation or vehicle for a given chemical must be specifically designed for that chemical to obtain maximum drug release. In formulating a cream or ointment, the physical properties of the drug and vehicle must be balanced in such a way as to provide optimum release. Specific additives may enhance or retard release of a chemical from the vehicle. For example, it has been shown that by simply incorporating a surfactant, release of chloramphenicol and tetracycline from an ointment formulation is enhanced.

A review of the broad problem of' release of drugs from topically applied dosage forms has been published by Grasso and Lansdown, "Methods of Measuring and Factors Affecting Percutaneous Absorption," Journal of Society of Cosmetic Chemists, 23:481-521, 1972.

In situations where filing an NADA ordinarily requires comparative bioequivalence testing of the subject drug with a like or similar formulation, for topically applied products CVM may, under certain conditions, determine that bioequivalence testing is not needed. When bioequivalence testing of topical/otic products is needed, see CVM's Bioequivalence Study Guideline, Section III. B.

The following, though not intended as an exhaustive list, are some of the inactive ingredients frequently used as vehicular components in topical/otic applications:

  • Lactose
  • Lanolin
  • Mineral oil
  • Petrolatum
  • Polyethylene glycol

VI. Discussion of Animal Models for Efficacy Testing

In dealing with the use of models of the infection/inflammation of minor wounds, burns, and abrasions, the differences and similarities of a specific model to naturally occurring infection/inflammation processes can be examined.

  1. Normal infection process.

    a. Microbial organisms occur on the skin at the time of injury, or are often carried there or transmitted from an infected source, such as a biting insect.

    b. A lesion occurs on the skin.

    c. The lesion provides the environmental situation required to allow the organisms that are present, or have been applied as the lesion is made, to reproduce and invade.

    d. An infected lesion results.

    e. The organisms causing the infection are present at the time of injury.


  2. Differences and similarities between natural and induced skin infection.

    a. In the natural infection, there is often a mixed population of microorganisms; in the model, only 1 or 2 types of microorganisms have been used. Whether model or natural infection, the new animal drug should be tested against the organism(s) considered to be primary pathogen(s).

    b. In the model system, the organisms against which the antibiotic/antifungal is acting are often those encountered in the normal surface flora. In the natural infection, in addition to the normal surface flora, other organisms (from exogenous sources) will likely be the infecting agents. These agents are the specific target of the applied antibiotic/antifungal.

    c. In the model system, organisms and active formulations may be inadvertently transmitted from one lesion to another, since artificially produced lesions may be made at close intervals on the skin.

VII. Classification, Testing Methodology, And Uses of Animal Models

A. Classification

Animal models may be developed in several ways to study antimicrobial agents. Some examples are as follows:

  1. Expanded flora test. This method can be used to evaluate broad spectrum antibiotic activity against large numbers of both gram-positive and gram-negative organisms induced by pretreatment occlusion. The expansion of flora that normally occurs when an occlusive wrap is applied to the skin is inhibited. A plastic wrap is applied for 48 hours prior to application of antibiotic agent. To be considered effective, a test material should destroy 99 percent of microorganisms. Bacterial counts will be low only if the antibiotic is active against both gram-positive and gram-negative organisms.
  2. Occlusion test. This test primarily estimates the bacteriostatic activity of an antibiotic against gram-positive microorganisms found on normal skin, showing how well the agent prevents a small number of bacteria from rapidly proliferating. Lesions are produced either by stripping with cellophane tape or by application of ammonium hydroxide, followed by inoculation with pathogenic organisms (usually staphylococci) and then covered by occlusive wrap. Lesions may be treated with a test compound after inoculation, and then inhibition of growth is observed or rapidity of healing is judged.
  3. Persistence test. This test determines the reservoir effect of the antibiotic, or its ability to bind to the stratum corneum to give a prolonged effect. In this test the antibiotic is usually applied 3 times daily for 3 days. After 3 days, these areas are occluded for 24 hours and then sampled for bacteria. Occlusion allows bacterial growth in the presence of the specific antibiotic being tested. The persistence of antimicrobial activity after application on the skin extends the time over which an antibiotic can exert an effect on a bacterial cell. This test is one means of measuring this characteristic in an animal model.
  4. Reduction of expanded flora produced by occlusive wrapping of the site. The inhibition of the expanded flora can be observed by application of antibiotics after sufficient time period of occlusion with the wrap. Prevention of expansion of the flora can also be used to test antibiotic activity. Antibiotic is applied to the wound or test site, and after continued occlusion, the inhibition of expansion of the flora is observed. The antimicrobial agent should eliminate the organism(s) from most lesions cultured within 18 hours after the agent is applied.

    A similar model may be used in which the stratum corneum is removed by producing blisters with ammonium hydroxide.

  5. In testing for prophylaxis the skin may be wrapped with occlusive plastic film for 48 hours either before, during, or following application of topical antimicrobial to determine the ability of the antimicrobial to suppress microbial proliferation stimulated by the moist, warm environment of occlusion.

B. Testing Methodology

Protocols should include methods to standardize the following variables in animal models:

  1. Location of contaminated wounds.
  2. Depth of incision.
  3. Type and quantity of inoculum.
  4. Method of inoculation.
  5. Time between inoculation and treatment.
  6. Method of culturing.
  7. Technique of treatment.
  8. Method of wound closure.
  9. System of grading infections.

The lesion is artificially made on the skin. Time is allowed for healing of the lesions (24 hours).

Pathogenic organisms are added to the lesion after 24 hours and allowed to remain an appropriate period of time before application of antibacterial/antifungal agents.

In some testing situations, the normally occurring flora are allowed to expand for varying times, producing successive replacement populations before the antibacterial/antifungal is applied. In some instances, the antibacterial/antifungal is added after 24-hour healing of the lesion to prevent expansion of the flora.

C. Animal Models for Skin Infection

Several animal models have been used to demonstrate the therapeutic and prophylactic effectiveness of topical antibacterials. In an extensive review of such animal models, Miller (Reference #1) describes some of these models, including: the production of abscesses in inoculated suture stitches in rats and guinea pigs; a rabbit ear-wound model; infection in inoculated and depilated rat skin; surgical wounds in the rat; a burn model inoculated with Pseudomonas   in the rat. In addition to rats, guinea pigs and rabbits, other test animals have included hamsters, mice, dogs and goats (References #1 - 13).

Other methods for attempting to justify the inclusion of antifungal and/or antiinflammatory components in topical/otic formulations are discussed in Section IX.

VIII. Inter-species Extrapolation and Dose Determination

The proper concentration of the active ingredient(s) and the proper vehicle of a topical/otic drug may be tentatively selected based on non-target species laboratory or clinical studies. (See Section IX, below.) Dose determination studies in the target species will not ordinarily be required if a therapeutically acceptable level of efficacy is demonstrated by controlled studies in the target species involving the concentration(s) of the active ingredient(s) and the vehicle tentatively selected from non-target species studies. There are anatomical and physico-chemical differences among the human and other animal species in the integument and external auditory canal. However, in view of the above stated variables in Section IV and the basic principles regarding vehicular components discussed in Section V, limited differences in the concentration of active ingredient(s) and/or in vehicular components may be only marginally significant or even essentially irrelevant.

Often, a topical/otic drug that is the subject of a new animal drug application (NADA) is identical to, or a pharmaceutical equivalent of, a drug that is approved under a new drug application (NDA) for use on humans. Unless the individual circumstances require otherwise, CVM would not ordinarily object to clinical testing in animals of a formulation that is identical to, or a pharmaceutical equivalent of, an NDA-approved formulation without dose determination studies having been conducted for the purpose of the NADA in non-target species or in the target species. Additionally, each/all component(s) in a formulation that is identical to or a pharmaceutical equivalent of an NDA-approved formulation may also be tested in clinical studies to establish "combination justification" for a veterinary drug, without the necessity for prior dose titration studies having been conducted in the target animal species.

IX. Testing Procedures for Combination Topical/Otic Preparations

There are essentially three applicable components in the Center's combination-drug policy: dose titration of the individual ingredients, testing for an optimal combination (i.e., checking for non-interference and synergism), and verification of the contribution of each constituent to the combination. See 21 CFR 514.1(b)(8)(v) and CVM's Guideline for Drug Combinations for Use in Animals.

The following is a schematic model that may be utilized in attempting to determine efficacy for a 3-way combination (antiinflammatory, antibacterial, antifungal) (see also Section VIII, above):

A. Non-Target Species Studies:

  1. Identify an optimum concentration (dose) and a vehicle that allows expression of the activity of the antifungal component, using a guinea pig/dermatophyte model; species other than guinea pig should be utilized when necessary to achieve an appropriate, adaptable model. The dermatophyte(s) used to induce the infections in the guinea pig will be pathogens known to infect the target species. Clinical isolates, not stock cultures, should be used.
  2. Using a bacterial challenge model in mice or other appropriate lab animal species, develop similar data for the antibacterial component. Again the pathogen used to induce the bacterial infection will be one known to infect the target species. Clinical isolates, not stock cultures, should be used.
  3. Using both of these models, demonstrate that the antimicrobial compounds are compatible -- not antagonistic and, if synergistic, have the proper ratio of the two to achieve desirable efficacy.
  4. Recognized laboratory studies can be employed to demonstrate the antiinflammatory effect and the relative potency of the antiinflammatory component, i.e., thymus involution, liver glycogen deposition, eosinopenia, carragheenan-induced edema response, etc. (Pharmacologic Techniques in Drug Evaluation, Nodine & Siegler, Chapter 69, pp. 491-500).
  5. Using the same two models (above items 1 and 2), demonstrate that the antiinflammatory agent does not interfere with the activity of either antimicrobial. It has been shown previously that hydrocortisone is compatible with an antibacterial for topical use. (Stritzler, C. & I. Frank, A.M.A. Arch. Dermat. 71 1955, p. 736).

B. Target Species Studies:

The proper concentrations of the active ingredients and the proper vehicle would be tentatively selected from the above studies for further testing in the target species. The following methodology would then be utilized to demonstrate the contribution of each active ingredient, and overall efficacy in the target species.

Group I - Appropriate control.

Group II - 0 antifungal + optimal antibacterial
+ optimal antiinflammatory.

Group III - 0 antibacterial + optimal antifungal
+ optimal antiinflammatory.

Group IV - 0 antiinflammatory + optimal antifungal
+ optimal antibacterial.

Group V - Optimal antifungal
+ optimal antibacterial
+ optimal antiinflammatory.

The necessary group comparisons to verify that each component is contributing to the combined efficacy of the product are as follows:

  1. The antifungal action of Group V (3-way combo.) must be superior to that of Group II to demonstrate the contribution of the antifungal agent.
  2. The antibacterial action of Group V must be superior to that of Group III to demonstrate the contribution of the antibacterial agent.
  3. The antiinflammatory action of Group V must be superior to that of Group IV to demonstrate the contribution of the antiinflammatory agent.
  4. Group V must relate favorably to Group I with respect to all three drug actions.

Conceptually, this methodology is appropriate for testing a new animal drug against both artificially induced model infections and naturally occurring infected cases in clinical field trials.

The same basic concepts described above to test efficacy for a 3-way combination may be appropriately reduced in scope for testing a product with a single active ingredient or a 2-way combination.

X. Summary

This guideline provides suggested directions for developmental design and scientific evaluation of effectiveness studies for topical/otic animal drugs containing any one or a combination of antimicrobial, antiinflammatory, and antifungal agents. The document outlines critical elements involved in establishing the rationale for a proposed product; defines topical categories; identifies variables affecting efficacy; discusses the influence of drug release from topical/otic preparations; provides classification, testing methodology, and uses for animal models; enunciates a practical perspective for assessing inter-species extrapolation and dose determination; and suggests testing procedures for combination topical/otic preparations.


(1) Miller, A., "In vivo Evaluation of Chemotherapeutic Substances," Advances in Applied Bacteriology, 14:151-183, 1971.

(2) Singleton, A.O., D. Davis, and J. Julian, "The Prevention of Wound Infection Following Contamination with Colon Organisms," Surgery, Gynecology, and Obstetrics, 108:389-392, 1959.

(3) Gray, F. J. and E. E. Kidd, "Topical Chemotherapy in Prevention of Wound Infection," Surgery, 54:891-899, 1964.

(4) Gingrass, R. P., A. S. Close, and E. H. Ellison, "The Effect of Various Topical and Parenteral Agents on the Prevention of Infection in Experimental Contaminated Wounds," Journal of Trauma, 4:763-783, 1964.

(5) Hopson, W. B., Jr., L. G. Britt, R. T. Sherman, and C. P. Ledes, "The Use of Topical Antibiotics in the Prevention of Experimental Wound Infection," Journal of Surgical Research, 8:261-266, 1968.

(6) Glotzer, D. J., W. S. Goodman, and L. H. Geronimus, "Topical Antibiotic Prophylaxis in Contaminated Wounds, "Archives of Surgery, 100:589-593, 1970

(7) Rosenthal, S. M., "Local and Systemic Therapy of Pseudomonas Septicemia in Burned Mice," Annals of Surgery, 165:97-103, 1967.

(8) Casten, D. F., R. J. Nach, and J. Spinzia, "An Experimental and Clinical Study of the Effectiveness of Antibiotic Wound Irrigation in Preventing Infection," Surgery, Gynecology, and Obstetrics, 118:783-787, 1964.

(9) Gastineau, F. M., and H. J. Florestano, "Clinical Experience with 'Polycin', a Polymyxin-Bacitracin Ointment, "Archives of Dermatology, 66:70-75, 1952

(10) Matsumoto, T., et al., "Antibiotic Topical Spray in a Simulated Combat Wound: II. Neomycin-Bacitracentsin-Polymyxin B., and Penicillin," Archives of Surgery, 96:786-789, 1968.

(11) Peterson, L. W., "Prophylaxis of Wound Infection - Studies with Particular Reference to Soap and Irrigation", Archives of Surgery, 50:177-183, 1945

(12) Knecht, A. T., Jr., H. M. Wise, Jr., F. B. Brinkley and R. P. Koch, "Bacitracin-Neomycin Detergent Spray: Effectiveness in the Treatment of Massive Experimental Wounds," Antibiotics Annual, 1958-1959:659-663, 1959.

(13) Duncan, W. C., M. E. McBride, and J. M. Knox, "Experimental Production of Infections in Humans," Journal of Investigative Dermatology, 54:319-323, 1970.


An Etiological and Epidemiological Exposition of Canine and Feline Otitis Externa

Is there scientific justification or medical rationale for multi-ingredient (antiinflammatory, antibacterial, antifungal) topical/otic preparations for use on dogs and/or cats? Is there an identifiable population of dogs and/or cats that would benefit from the use of the above described fixed 3-way drug combination? The following information addresses the immediately preceding issues.

(1) Etiology: The cause of otitis externa is complex. Many factors have been incriminated, but the exact etiology has not been determined. It has been suggested that by the time the animal is presented for treatment the initial cause may not be apparent.

Factors associated with otitis externa are foreign bodies, matted hair, dried wax, trauma to the ear from cleaning and examination, ectoparasites, allergic response (generally associated with other allergic skin problems), bacterial and fungal flora in the ear and excessive moisture. Probably the cause of otitis externa is a combination of these factors. All the factors listed produce irritation to the ear, which predisposes to the development of otitis externa (Current Vet. Therapy, Kirk, VI Ed. 1977, Section 9, p 850).

(2) The most frequent isolates from the ears of dogs and cats are yeasts of the Pityrosporon spp.  and bacteria of the Proteus spp., Pseudomonas spp.,  and Staphylococcus spp.   It is felt that Proteus spp.  and Pseudomonas spp.  can be primary pathogens in the disease. However, Staphylococcus spp.  may be found in normal ears and may grow only in response to inflammation initiated by some other factor.

Pityrosporon  is considered by some investigators to be a primary pathogen and by others to be similar to Staphylococcus.  If a pure culture of Pityrosporon  is isolated, it may be considered to be the causative factor but could also be a secondary pathogen. Often there will be mixed cultures from the ears, especially in chronic cases (Current Vet. Therapy, Kirk, VI Ed. 1977, Section 9, pp 850-51).

(3) The following summarizes relevant points relating to the microorganisms occurring in conjunction with otitis externa in dogs and cats.

Submit Comments

You can submit online or written comments on any guidance at any time (see 21 CFR 10.115(g)(5))

If unable to submit comments online, please mail written comments to:

Dockets Management
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Rockville, MD 20852

All written comments should be identified with this document's docket number: FDA-2021-D-0625.


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