CVM VMACExperience with 558.15 Studies

Thomas R. Shryock, Ph.D.

Elanco Animal Health

January 23, 2002

Discussion Outline• 558.15 Study Protocol Review• Actual experience with 558.15 salmonella

shedding studies• Limitations of salmonella shedding studies• Lessons learned • Relevance to Proposed Pathogen Load

studies• Conclusions

21 CFR 558.15 Studies1970 FDA Task Force report rationale to test test in-feed antimicrobials for effects on salmonella Human illnesses and death have been reported due to bothantibiotic-sensitive and antibiotic-resistant bacteria of animalorigin. Food-producing animals constitute a major reservoirof certain bacteria (e.g. salmonella) pathogenic for man.Evidence suggests that the use of certain antibiotics in food-producing animals promotes an increase in the animalreservoir of salmonella through promotion of cross-colonization and infection, prolongation of the carrier state,and relapse of disease. Furthermore, the use of someantibiotics in animals produces a marked increase in theprevalence of R-factor containing bacteria which may betransmissible to man’s enteric flora. These observations leadto the logical conclusion, though not fully documented, thatsuch practices give rise to a human health hazard.

21 CFR 558.15 StudiesObjectives

• For salmonella shedding studies a sponsor must show:

• drug does not adversely impact quantities, prevalence or duration of salmonella shed (over baseline non-med control).

• Drug does not increase salmonella or coliform resistance (over baseline non-med control) to drugs used in either human or animal medicine

• 558.15 regulation gives no specifics in study design or expectations for the type of data to be submitted for CVM review. Sponsors were to consult with CVM

Industry Experience with Conducting 558.15 Studies

• FDA required for feed additive for > 14 days• Salmonella shedding (Quantities, Prevalence, Duration) +

tissues at study end• Antibiotic resistance of salmonellae and coliforms

• Most studies done by C.A.R.E. (Dr. Fagerberg)• Initially a single study to assess all parameters• Evolved to salmonella study + separate coliform study• Currently 3 studies:

• Salmonella Quantitation (high dose challenge)• Salmonella Prevalence & Duration (lower challenge dose)• Coliform Resistance Study

General Study Timeline

Day -14 -7 -1 0 1 4 7 14 21 28 35 42 49 56Culture X X X X X X X X X X X X XWeigh X XIntake X X X X X X X X X X X XClin ObX X X X X X X X X X X X X XNecropsy X

liver, spleen, MLN, cecal

Note: Quant-4 week study, Prev/Dur 8 week study

Fast 12-24h Challenge 2X Feed (Med or Non-med)

High Challenge 1011 cfuLow Challenge 106 cfu

General Salmonella Study DesignAnimals-SPF or Salmonella-free, <20% baseline resistance

in E. coliSalmonella-NalR strain specific to animal species on testAntibiotics-12 (AG, b-lactams, phenicol, quinolone, sulfa, tet)

MICs determined Feed- Requires validated analytical feed assay

Requires dose from efficacy studiesRequires formulated premix product

Study Groups10-12 animals/group, housed individuallyMed, Env. Control, Non-med. Separate caretakers, strict biosecurityIsolation cage - minimal coprophagia

GLP Study

Industry Experience with 558.15 Studies

Discovery Development Product Support

Approval

Registration

Activities

Sales

Find New Technolog

y

A.Target I.D. Screen Dev

B. Screen Opt Early SAR

C. New Screens

Lead Optimizatio

nA.Select Final

ChemistryB.FormulationC.Candidate

SelectionD. Toxicology

Product Development

• Non Clinical• Clinical• Data for CM&C

Submission

558.15 studies

Interpretation IssuesPass-Fail for Candidate

• Statistical significance vs. biological significance interpretation evolved from Larry Rollins, CVM

• Pass if med. Vs. non-med. Groups had:• Salm Quantitation < 1-2 logs difference

• Salm Prevalence <20-30% difference

• Salm Duration <7-10 days difference

• Antibiotic Resistance Moot Point (no differences ever observed)

• Unclear if all 3 salmonella parameters, or just 2, had to be within "bounds" to Pass the molecule

• No scientific evidence that these criteria actually relate to farms, meat, or illness

FIGURE 3. QUANTITATION OF SALMONELLA IN FECAL SAMPLES

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50 55 60

DAYS

log

10

CF

U/g

ram

Medicated

Non-Medicated

**

Table 1. Prevalence and Duration of Salmonella Shedding

Treatment PigPost-Challenge Sampling Day* Proportion

of DaysDuration ofShedding

Group No. 2 4 7 9 11 14 21 28 35 42 49 56 Positive (Days)

Treated 354 1 1 1 1 1 0 0 0 0 0 0 0 .417 11358 1 1 - - - - - - - - - - - -365 1 1 0 0 0 0 0 0 1 1 1 0 .417 49366 1 1 1 1 - - 1 - 1 1 - 1 .667 56375 1 1 1 0 1 1 1 0 1 1 0 0 .667 42377 1 1 1 1 0 1 1 0 0 1 1 0 .667 49379 1 1 1 1 0 1 1 1 0 1 1 1 .833 56392 1 1 0 0 0 0 0 0 0 0 0 0 .167 4395 1 1 1 1 1 1 1 0 1 1 0 0 .750 42398 1 1 1 0 1 1 0 1 0 0 0 0 .500 28

ProportionPositive

1.0 1.0 .778 .556 .444 .556 .556 .222 .444 .667 .333 .222 .565

Non- 355 1 1 0 0 0 0 0 0 0 0 1 0 .250 49medicated 357 1 1 1 1 1 1 1 0 0 1 1 0 .750 49Control 360 1 1 1 1 0 0 0 1 1 1 1 0 .667 49

361 1 1 1 1 1 1 1 0 1 1 1 1 .917 56370 1 1 1 0 0 0 0 0 0 1 0 1 .417 56389 1 1 1 1 1 1 1 0 1 1 1 1 .917 56390 1 1 1 1 1 1 1 1 1 1 1 1 1.00 56391 1 1 1 1 1 1 0 0 0 0 1 1 .667 56393 1 1 0 1 0 0 0 0 0 0 1 1 .417 56400 1 1 0 1 1 1 0 0 0 0 1 0 .500 49

Proportion 1.0 1.0 .700 .800 .600 .600 .400 .200 .400 .600 .900 .600 .650Positive

*1 = Indicates salmonella presence, 0 = no salmonella were detected, - = no sample available.

Industry Experience with Conducting 558.15 Studies

• Most drugs tested were G+ active, all in-feed

• Baseline resistance (<20% rule) in coliforms in test animals is difficult to achieve

• 73% Pass rate; variety of antibiotic classes

• Failure of studies may be due more to interpretation of "borderline" data than a biologically significant "shedding"

• A failed study could be repeated and be passed

• If Pass in one species, usually pass in others

• "Study Creep"

• One initial study became 3 separate studies

• Separate series of studies for each species of use

Study Limitations• Relationship of test model results to commercial farms

has not been established• Relationship of test model results to carcass

contamination levels was not established• Relationship of test model results to public health was not

established• Only one strain of salmonella tested for "shedding"

• Salmonella given prior to medication would sequester intracellularly, bypassing normal flora (minimized by fasting).

• High-challenge dose is unrealistic exposure• Overcomes protective flora, causes disease• Sick animals don't eat as much

Model Study vs. Field Situation

• The field situation is considerably different:• Animals previously exposed (immune) or no salmonella

present • Natural, continuous exposure, often at young age• Small proportion of herd/flock culture positive, many

undetected carriers• Antibiotic superimposed in presence of low levels of

shedding• Antibiotic administered to diseased and stressed animals• Housing, biosecurity, environment are not "research

grade" status

• Model study cannot "factor in" these real world situations, nor predict what will happen there.

558.15 Lessons to apply to Proposed Pathogen Load Studies• Only database so far is for in-feed medication for multi-

week studies with salmonella challenge• No experience with higher doses, other routes, shorter

durations or post-medication withdrawal effects• Therapeutic drugs usually given weeks to months before

slaughter. When to sample? Relevance?• No experience with other foodborne pathogens-will

multiple studies be needed ("study creep")?

• Very little experience with broad-spectrum drugs• G- spectrum likely to decrease shedding!

• Biological variation occurs in individual animals; results may not always be clear-cut for interpretation.

Other Lessons to apply to Proposed Pathogen Load Studies• CVM Pre-approval Workshop groups found no value to

conducting Pathogen Load studies• Exponent Report found no consistent evidence to indicate

an association of salmonella shedding with antibiotics• FDA Task Force recommendations did not include

therapeutic drugs• A standardized, validated protocol, with clear interpretation

of results does not exist for Pathogen Load studies. A modify-as-you-go approach, as was done for 558.15 studies, is unacceptable. • De facto implementation of Pathogen Load studies has

already begun as CVM is asking sponsors to conduct these studies for therapeutic drugs

Pathogen Load Study Relevance• 8% of US Swine salmonella positive, HACCP pork

carcass baseline is 8%. Feed additive use is 90%.• The magnitude of salmonella for these food animal

species is already quite low.

• How would pre-approval pathogen load studies on therapeutic uses of new antibiotics be relevant?

• Six of top ten human salmonella serotypes differ from those recovered from food animals • How can Pathogen Load studies be designed to be

predictive of public health impact?

Pathogen Load Study Relevance• Salmonella "load" in intestinal tract increases from farm to

slaughter plant; different serotypes emerge• Pathogen load is more affected near slaughter by

transport, stress, feed, environment than prior antibiotic use.

• Contamination post-slaughter can occur but on-farm origin assumed

• Effective interventions occur at/after slaughter plant (HACCP, cooking, etc.) to minimize foodborne pathogen contamination of carcasses• Muscle is sterile upon arrival- meat is contaminated upon

cut-up so containment is especially important

Conclusions• 558.15 remains in place for in-feed antibiotic

products• Pathogen Load studies for therapeutic

antibiotics cannot be designed to have relevance to on-farm practices, meat contamination and subsequent human illness

• Pathogen Load study requirements are an unnecessary impediment to the development of new food animal therapeutic antibiotics.