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With regard to antiseptics, what is "persistence", what is about ExCel_AP^® that makes it persistent, and what data is available to suppor this?

In its tentative final monograph (TFM) entitled Topical Antimicrobial Drug Products for Over the Counter Human Use: Tentative Final Monograph for Health-Care Antiseptic Drug Products,¹ the Food and Drug Administration (FDA), defines a patient preoperative skin preparation as “a fast-acting [rapidly kills microorganisms], broad-spectrum [kills a wide variety of microorganism species], persistent antiseptic-containing preparation that significantly reduces the number of microorganisms on intact skin.”^1(p31442)

In this context, “persistence” is defined as the suppression of regrowth of microorganisms on treated areas.  As soon as an area of the skin is treated with an antiseptic, recovery and replication of the surviving microorganisms begins; the more persistent a given antiseptic product is, the slower the rate of recovery and replication will be.  While somewhat arbitrary, an antiseptic is generally considered to be persistent as long as it is able to maintain the count of bacteria on the skin below the original “baseline” count determined before treatment.  In other words, the product is considered to be persistent as long as a percent or log,₁₀ reduction (from the original baseline count) can be demonstrated.

An antiseptic’s persistence is dependent on a variety of factors including:

1. The concentration of the active ingredient(s). In most cases, the higher the concentration of active ingredients, the greater the log₁₀ reduction in bacterial count, and the more regrowth will need to take place before the counts come back to original baseline levels.

2. The presence or absence of either ethyl alcohol (EtOH; 60%-95%) or isopropyl alcohol (IPA; 70%-91%). The CDC’s most recent guidance for the prevention of surgical site infection, states that alcohol is “the most effective and rapid-acting skin antiseptic.”

In addition to being fast acting and substantive, contrary to common understanding, EtOH and IPA are also quite persistent – not because they remain on the skin in contact with whatever bacteria are present, but instead, because they render those organisms that survive exposure to them extremely limited in their ability to replicate. 

As illustrated in the table below taken from FDA’s Microbiology Review of ChloraPrep’s New Drug Application (see http://www.fda.gov/cder/foi/nda/2000/20-832_CHLORAPREP%20ONE-STEP%20ANTISEPTIC_microbr.pdf), alcohol is as persistent as ChloraPrep® for 24 hours.  The table is followed with a graphical representation of the same data.  Overlap of 95% confidence intervals indicates no significant difference between the products tested.

3. The presence of inactive ingredients such as film-forming agents can cause antiseptic ingredients to remain in contact with the skin for longer periods of time resulting in greater initial log₁₀ reductions.  Similarly, thickening agents can lead to less of the formulation running off the treatment area before killing any microorganisms, and in turn, lead to greater initial log₁₀ reductions as well.

4. The time the antiseptic formulation is in contact with the skin (ie, the application time). The longer the antiseptic is in contact with the area being treated, the greater initial log₁₀ reductions, and in turn, greater persistence.

5. The manner in which the formulation is applied (eg, firm scrubbing vs. gentle painting; back and forth motion vs. circular motion). More aggressive scrubbing allows for greater penetration of the antiseptic into deeper layers of the skin where some microorganisms exist – thus greater initial log₁₀ reductions, and in turn, greater persistence.

6. The abrasiveness of foam applicators can affect the extent to which an antiseptic penetrates into deeper layers of the skin with the same level of force applied.  This too can lead to greater initial log₁₀ reductions, and in turn, greater persistence.

Simply put, ExCel_AP is persistent because it contains two persistent antiseptic ingredients

• 7.5% (w/w) povidone-iodine³
• 72% (v/v) isopropyl alcohol – see above

The persistence of these ingredients is further amplified by the presence of an acrylate film former which serves to keep povidone-iodine molecules in contact with the skin for a longer period of time.

In support of these assertions, the table and graph below shows the persistence of ExCel_AP when delivered using Covidien’s MERLIN™ applicator.  ExCel_AP maintained a 1 log₁₀ reduction 72 hours after application – virtually identical to the 4% CHG-containing formulation Hibiclens®.

The graph below shows the persistence of ExCel_AP when delivered using triple swabsticks.  It was also shown to maintain a 1 log₁₀ reduction 7 days hours after application – not significantly different from the 2% CHG-containing formulation ChloraPrep®.

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