Missing the SPF "Target"

If you have ever ordered sunscreen testing you were more than likely requested to nominate the “TARGET” SPF. I have never been a fan of requiring this to be reported on SPF study reports as is now a mandatory part of ISO 24444 and by implication, AS/NZS 2604 [1] for Australia and soon, New Zealand.

Historically, this target requirement originates in the USA and was created with the intent of setting the exposure range for the IV light challenge. However, there are a number of reasons why this may not be appropriate…

A. If this target is relied on for the test subject/s run on the first day of testing, then there is going to be a high chance that these subjects will be over-exposed. ?As well as resulting in an invalid result, this is obviously not best clinical practice for safety of SPF test volunteers. As well, the persistence of the resulting tan will mean that these subjects will most likely be excluded from future testing for many months.

B. Nominating a Target has been shown to bias the results. Evidence of this has previously been presented to FDA by several major marketers. The determination of bias can fairly easily be determined for any particular test facility simply by nominating high and low Target SPFs for the same batch of the same formulation. Running the test on the same test subjects will be even more supportable!

C. Whilst there is obvious concern for over-estimation of expected SPF, the reverse is often the case i.e. conservativism.? Fig 1 shows data collected from one test lab.? The red line indicates the nominated target (or midpoint of a range). [note that this line in not linear as multiple samples nominate the same SPF Target, so these appear as plateaus, especially for SPF category cut-offs of 15,30,50 and 60 (50+). As can be seen, there is large variation in best guess estimating, both over and under.

Fig 1. Expected versus Actual SPF

D. 10 in 1 day testing, that is, running a complete study on the same day is still offered by some laboratories. In this case, there is no capacity to check and adjust and in effect, if actually true, the test has really been run in a “pass- mark “approach, rather than attempting to arrive at an actual value. By this, I mean that all results showing say SPF 57 might actually be as high as 80, in which case the manufacturer might be wasting substantial money on excess UV active content – far more than the cost of a retest!?

E. Regulators have, in the past, pounced on reports where the target SPF varies from the actual SPF result. For example, a conservative target SPF of say 55 (which would categorise as “High Protection” might achieve an SPF of say 62, which would categorise as “VERY High Protection”. In this case, conservative estimation has resulted in an uplift of claim permission which usually delights sunscreen marketers.

Those skilled in the art will be well aware that missing the target can result in lengthy discussion on the validity of test results compared with expectations.

So, how do we approach this in a different way? ?Evidence from the yield norms for UV actives points to a reasonable way to approach the “target”. These norms are available on line in the form of in silico SPF estimation by BASF and DSM. [2,3] They range around 2 SPF units for each percentage of active included.

Fig 2 shows this yield from a survey the author conducted for TGA Listed sunscreens. This would indicate that very few achieved greater than 3 (even if the reports were authentic).

Fig 2. SPF Yield per percentage actives for SPF 50 and 50+ categories Listed by TGA.

When the samples are presented to the test lab as improvements by adjustments to a previous formulation which has a determined in vivo SPF, then this is more likely to be reliable – but is not always the case. Of course, the excipients, boosters and the nature of the formulation do have an impact. Mainly, this is as a result of dried down film thickness on the skin.

So, what is my rule of thumb?

1.???? Determine the total content of active and multiply by 2 to 3 at most.

2.???? If the product is a volatile liquid, reduce this estimate.

3.???? If the product is non-aqueous type, multiply by 2

4.???? Look at the history of similar formulations.

5.???? From the test lab, be patient and look for a feedback loop from 3 to 5 test subjects before going for the full 10 or more subject test.

References

1.???? AS/NZS 2604 Sunscreen Standard 2021. https://store.standards.org.au/product/as-nzs-2604-2021

2.???? BASF Sunscreen Simulator? https://sunscreensimulator.basf.com/Sunscreen_Simulator/login?

3.???? DSM Sunscreen Optimiser https://www.sunscreen-optimizer.com/index.html