S10 Photosafety Evaluation   
of Pharmaceuticals 
 
Guidance for Industry 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U. S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
Center for Biologics Evaluation and Research (CBER) 
 
January 2015 
ICH
   
S10 Photosafety Evaluation   
of Pharmaceuticals 
 
Guidance for Industry 
 
 
 
 
 
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Food and Drug Administration 
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ocod@fda.hhs.gov 
http://www.fda.gov/BiologicsBloodVaccines/GuidanceComplianceRegulatoryInformation/Guidances/default.htm   
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
Center for Biologics Evaluation and Research (CBER)  
 
January 2015 
ICH 
  Contains Nonbinding Recommendations 
 
TABLE OF CONTENTS 
 
 
I.  INTRODUCTION (1) ....................................................................................................... 1 
A.  Objectives of the Guidance (1.1) ................................................................................................... 1 
B.  Background (1.2) ............................................................................................................................ 2 
C.  Scope of the Guidance (1.3) ........................................................................................................... 2 
D.  General Principles (1.4) ................................................................................................................. 3 
II.  FACTORS TO CONSIDER IN THE PHOTOSAFETY EVALUATION (2) ............. 3 
A.  Photochemical Properties (2.1) ..................................................................................................... 3 
B.  Tissue Distribution/Pharmacokinetics (2.2) ................................................................................ 4 
C.  Metabolite Considerations (2.3) .................................................................................................... 5 
D.  Pharmacological Properties (2.4) ................................................................................................. 5 
III.  NONCLINICAL PHOTOSAFETY TESTS (3) ............................................................. 5 
A.  General Considerations (3.1) ........................................................................................................ 5 
B.  Photoreactivity Tests Using Chemical Assays (3.2) .................................................................... 6 
C.  Phototoxicity Tests Using In Vitro Assays (3.3) .......................................................................... 6 
D.  Photosafety Tests Using In Vivo Assays and Systemic Administration (3.4) ........................... 7 
E.  Photosafety Tests Using In Vivo Assays and Dermal Administration (3.5) .............................. 9 
IV.  CLINICAL PHOTOSAFETY ASSESSMENT (4) ........................................................ 9 
V.  ASSESSMENT STRATEGIES (5) ................................................................................ 10 
A.  Recommendations for Pharmaceuticals Given via Systemic Routes (5.1) .............................. 10 
1. Assessment of Phototoxicity Potential (5.1.1) ................................................................................ 10 
2. Evaluation of Phototoxicity (5.1.2) ................................................................................................ 10 
B.  Recommendations for Pharmaceuticals Given via Dermal Routes (5.2) ................................ 11 
1. Assessment of Phototoxicity Potential (5.2.1) ................................................................................ 11 
2. Experimental Evaluation of Phototoxicity and Photoallergy (5.2.2) ............................................. 11 
VI.  ENDNOTES (6) ............................................................................................................... 14 
VII.  GLOSSARY (7) ............................................................................................................... 16 
VIII.  REFERENCES ................................................................................................................ 18 
  Contains Nonbinding Recommendations 
 
S10 Photosafety Evaluation of Pharmaceuticals 
1
Guidance for Industry  
 
 
This guidance represents the Food and Drug Administrations (FDAs) current thinking on this topic.  It 
does not create or confer any rights for or on any person and does not operate to bind FDA or the public.  
You can use an alternative approach if the approach satisfies the requirements of the applicable statutes and 
regulations.  If you want to discuss an alternative approach, contact the FDA staff responsible for 
implementing this guidance.  If you cannot identify the appropriate FDA staff, call the appropriate number 
listed on the title page of this guidance.  
 
 
 
 
 
I.  INTRODUCTION (1)2 
  
A.  Objectives of the Guidance (1.1) 
 
The purpose of this guidance is to recommend international standards for photosafety assessment, 
and to harmonize such assessments that support human clinical trials and marketing authorizations 
for pharmaceuticals.  This guidance includes factors for initiation of and triggers for additional 
photosafety assessment and should be read in conjunction with ICH M3(R2), section XIV(14) on 
Photosafety Testing (Ref. 1).3  It should reduce the likelihood that substantial differences in 
recommendations for photosafety assessment will exist among regions. 
 
This guidance is divided into several sections: 
  
  Section II(2) discusses factors to consider in any evaluation of photosafety.  
  Section III(3) describes existing nonclinical photosafety tests, but this section does not 
describe specific testing strategies.  
  Section IV(4) mentions clinical photosafety assessment.  
                                                 
1
 This guidance was developed within the Safety Implementation Working Group of the International Conference on 
Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) and has been 
subject to consultation by the regulatory parties, in accordance with the ICH process.  This document has been 
endorsed by the ICH Steering Committee at Step 4 of the ICH process, November 2013.  At Step 4 of the process, the 
final draft is recommended for adoption to the regulatory bodies of the European Union, Japan, and the United States. 
  
2 Arabic numbers reflect the organizational breakdown of the document endorsed by the ICH Steering Committee at 
Step 4 of the ICH process, November 2013. 
 
3 We update guidances periodically.  To make sure you have the most recent version of a guidance, check the FDA 
Drugs guidance Web page at 
http://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/default.htm.    
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  Section V(5) provides strategies for determining how to assess photosafety for drugs given 
by routes intended to produce systemic exposure or by the dermal route using the 
considerations and tests described in sections II(2), III(3), and IV(4).  
 
Consideration should be given to the use of non-animal methods or clinical data for photosafety 
assessment, which could reduce the use of animals in accordance with the 3R 
(replacement/reduction/refinement) principles. 
 
FDAs guidance documents, including this guidance, do not establish legally enforceable 
responsibilities.  Instead, guidances describe the Agencys current thinking on a topic and should 
be viewed only as recommendations, unless specific regulatory or statutory requirements are cited.  
The use of the word should in Agency guidances means that something is suggested or 
recommended, but not required.  
  
B.  Background (1.2) 
 
The ICH M3(R2) guidance provides certain information regarding timing of the photosafety 
assessment relative to clinical development.  It recommends that an initial assessment of 
phototoxicity potential be conducted and, if appropriate, an experimental evaluation be undertaken 
before exposure of large numbers of subjects (phase 3).  Similarly, ICH S9 (Ref. 2) describes the 
timing of photosafety testing for oncology products.  However, neither ICH M3(R2) nor ICH S9 
provides specific information regarding testing strategies.  This ICH S10 guidance outlines 
further details on when photosafety testing is warranted, and on possible assessment strategies. 
 
C.  Scope of the Guidance (1.3) 
  
This guidance generally applies to new active pharmaceutical ingredients (APIs), new excipients, 
clinical formulations for dermal application (including dermal patches), and photodynamic 
therapy products. 
 
Specific guidance for pharmaceuticals given via ocular routes is not provided because the 
reliability of in vitro approaches in predicting ocular phototoxicity is unknown, and there are no 
standardized in vivo approaches for assessing phototoxicity for products administered via the 
ocular routes (see Note 1). 
 
Photodynamic therapy drugs are developed with photochemical reactivity as an inherent aspect of 
their intended pharmacology, and additional assessment of their phototoxicity is not usually 
warranted.  However, an evaluation of the toxicokinetics and tissue distribution of photodynamic 
therapy drugs is warranted to enable appropriate risk management in patients. 
  
This guidance does not generally apply to peptides, proteins, antibody drug conjugates, or 
oligonucleotides.  Furthermore, this guidance does not apply to components of marketed products 
unless there is a new cause for concern for either the API or an excipient (e.g., a reformulation 
from a tablet to a topical cream). 
 
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D.  General Principles (1.4) 
  
The photosafety assessment of a pharmaceutical is an integrated process that can involve an 
evaluation of photochemical characteristics, data from nonclinical studies, and human safety 
information.  The photosafety assessment aims to determine whether risk minimization measures 
are warranted to prevent adverse events in humans. 
   
Four different effects have been discussed in connection with photosafety testing:  phototoxicity, 
photoallergy, photogenotoxicity, and photocarcinogenicity.  Testing for photogenotoxicity (Note 
2) and photocarcinogenicity (Note 6 of ICH M3(R2)) is not currently considered useful for human 
pharmaceuticals.  This guidance addresses only phototoxicity and photoallergy effects as defined 
below: 
 
  Phototoxicity (photoirritation):  An acute light-induced tissue response to a photoreactive 
chemical. 
 
  Photoallergy:  An immunologically mediated reaction to a chemical, initiated by the 
formation of photoproducts (e.g., protein adducts) following a photochemical reaction. 
 
Photosensitization is a general term occasionally used to describe all light-induced tissue 
reactions.  However, in order to clearly distinguish between photoallergy and phototoxicity, the 
term photosensitization is not used in this guidance. 
 
For a chemical to demonstrate phototoxicity and/or photoallergy, the following characteristics are 
critical: 
  
  absorbs light within the range of natural sunlight (290-700 nm) 
  generates a reactive species following absorption of UV-visible light 
  distributes sufficiently to light-exposed tissues (e.g., skin, eye) 
 
If one or more of these conditions are not met, a compound will usually not present a concern for 
direct phototoxicity.  However, increased sensitivity of skin to light can also occur through 
indirect mechanisms.  Such mechanisms are not generally addressed by the testing outlined in this 
guidance (see also section II.D(2.4)). 
 
 
II.  FACTORS TO CONSIDER IN THE PHOTOSAFETY EVALUATION (2) 
 
A.  Photochemical Properties (2.1) 
 
The initial consideration for assessment of photoreactive potential is whether a compound absorbs 
photons at any wavelength between 290 and 700 nm.  A compound that does not have a molar 
extinction coefficient (MEC) greater than 1000 L mol-1 cm-1 at any wavelength between 290 and 
700 nm (Ref. 3) is not considered to be sufficiently photoreactive to result in direct phototoxicity 
(see Note 3 for further details). 
 
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  Contains Nonbinding Recommendations 
 
Excitation of molecules by light can lead to generation of reactive oxygen species (ROS), 
including superoxide anion and singlet oxygen via energy transfer mechanisms. Although 
photoreactivity can result in other molecular outcomes (e.g., formation of photoadducts or 
cytotoxic photoproducts), even in these cases, it appears that ROS are typically generated as well.  
Thus, ROS generation following irradiation with UV-visible light can be an indicator of 
phototoxicity potential. 
 
Photostability testing (see ICH Q1B (Ref. 4)) can also suggest the potential for photoreactivity.  
However, not all photoreactive compounds are detected under these conditions, and 
photodegradation per se does not imply that a drug will be phototoxic.  Therefore, photostability 
testing alone should not be used to determine whether further photosafety evaluation is warranted. 
 
Assessments of photochemical properties should be conducted using high-quality scientific 
standards with data collection records readily available, or in compliance with good laboratory 
practice/good manufacturing practice regulations. 
 
B.  Tissue Distribution/Pharmacokinetics (2.2) 
 
The concentration of a photoreactive chemical in tissue at the time of light exposure is a very 
important pharmacokinetic parameter in determining whether a phototoxic reaction will occur.  
This concentration depends on a variety of factors, such as plasma concentration; perfusion of the 
tissue; partitioning from vascular to interstitial and cellular compartments; and binding, retention, 
and accumulation of the chemical in the tissue. The duration of exposure depends upon clearance 
rates as reflected by half lives in plasma and tissue. Collectively, these parameters define the mean 
residence time of the photoreactive chemical in tissue. 
 
Binding, retention, or accumulation of a compound in a tissue is not critical for a phototoxic 
reaction.  If a molecule is sufficiently photoreactive, it might produce a phototoxic reaction at the 
concentration achieved in plasma or interstitial fluid.  However, compounds having longer 
half-lives in plasma, longer mean residence time in sun-exposed tissues, or higher tissue to plasma 
concentration ratios are more likely to produce a phototoxic reaction than compounds with shorter 
half-lives, shorter residence times, or lower tissue to plasma ratios.  Furthermore, the longer the 
concentration of a compound is maintained at a level above that critical for a photochemical 
reaction, the longer a person is at risk for phototoxicity. 
 
Although a tissue concentration threshold below which the risk for phototoxic reactions would 
be negligible is scientifically plausible, there are currently no data to delineate such generic 
thresholds for all compounds.  Nevertheless, on a case-by-case basis, it can be possible to 
justify that further photosafety assessment is not warranted based upon actual or anticipated 
tissue drug levels in humans, and taking into consideration the factors discussed above.   
Examples could include (1) a drug for which overall systemic exposure levels are very low or (2) 
a drug with a very short plasma half-life or tissue residence. 
 
Compound binding to tissue components (e.g., melanin, keratin) is one mechanism by which tissue 
retention and/or accumulation can occur.  Although melanin binding can increase tissue levels, 
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  Contains Nonbinding Recommendations 
 
experience with melanin binding drugs suggests such binding alone does not present a photosafety 
concern. 
 
A single-dose tissue distribution study, with animals assessed at multiple timepoints after dosing, 
will generally provide an adequate assessment of relative tissue to plasma concentration ratios, 
tissue residence time, and the potential for retention and accumulation. Assessment timepoints 
should be appropriately spaced in such a study to account for the drug half-life.  
 
Compounds activated by visible light and exhibiting long elimination half-lives in internal tissues 
have been demonstrated to cause injury to those tissues if exposed to intense light during medical 
procedures.  Consequently, for those compounds activated by visible light with potent in vivo 
phototoxicity or known to be phototoxic based on their mechanism of action, such as 
photodynamic therapy drugs, distribution to internal tissues should be measured and 
tissue-specific half-lives estimated.  Drugs that only absorb ultraviolet (UV) light or have short 
tissue elimination half-lives are not likely to present a risk to internal tissues even if they are 
known to be photoreactive. 
 
C.  Metabolite Considerations (2.3) 
 
Metabolites generally do not warrant separate photosafety assessments, because metabolism does 
not typically result in chromophores that are substantially different from those in the parent 
molecule. 
  
D.  Pharmacological Properties (2.4) 
   
In many cases, drug-induced phototoxicity is due to the chemical structure and not the 
pharmacology.  However, certain pharmacologic properties (e.g., immunosuppression, 
perturbation of heme homeostasis) can enhance susceptibility to light-induced effects, such as skin 
irritation or UV-induced skin tumor formation.  The testing strategies outlined in this document 
are not designed to detect these types of indirect mechanisms.  Some of these indirect 
mechanisms can be identified and evaluated in other nonclinical pharmacology/toxicity testing; 
however, phototoxicity related to other indirect mechanisms might only become apparent with 
human experience. 
 
  
III.  NONCLINICAL PHOTOSAFETY TESTS (3) 
 
A.  General Considerations (3.1) 
 
Carefully selected conditions that consider both the model system and exposure to a relevant 
radiation spectrum are critical for nonclinical photosafety testing.  Ideally, a nonclinical assay 
should exhibit both high sensitivity and specificity (i.e., low false negative and low false positive 
rates).  However, to support the assessment strategies described in this document, it is most 
important that nonclinical photosafety assays show high sensitivity resulting in a low frequency of 
false negatives (i.e., a high negative predictive value).  This is because negative assay results 
usually do not warrant further photosafety evaluation.  The available nonclinical assays, both in 
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  Contains Nonbinding Recommendations 
 
vitro and in vivo, are focused primarily on detecting potential phototoxicity, which might or might 
not translate into clinically relevant phototoxicity. 
 
Selection of irradiation conditions is critical for both in vitro and in vivo assays.  Natural sunlight 
represents the broadest range of light exposure that humans might be exposed to regularly.  
However, sunlight per se is not well defined and depends on many factors, such as latitude, 
altitude, season, time of day, and weather.  In addition, sensitivity of human skin to natural 
sunlight depends on a number of individual factors (e.g., skin type, anatomical site and tanning 
status).  Standardized sunlight exposure conditions have been defined by various organizations.  
Such standards (e.g., CIE-85-1989 (Ref. 5)) should be considered in order to assess suitability of a 
sunlight simulator light source, and irradiance and irradiation dose should be normalized based on 
the ultraviolet light A (UVA) part of the applied spectrum.  UVA doses ranging from 5 to 20 
J/cm2 are successfully used in current in vitro and in vivo phototoxicity assays.  These UVA doses 
are comparable to those obtained during prolonged outdoor activities on summer days around 
noon time, in temperate zones, and at sea level.  In humans, sunburn reactions caused by 
ultraviolet light B (UVB) normally limit total sunlight exposure.  In nonclinical phototoxicity 
assays, however, the amount of UVB should not limit the overall irradiation and might be 
attenuated (partially filtered) so that relevant UVA doses can be tested without reducing assay 
sensitivity.  Penetration of UVB light into human skin is mainly limited to the epidermis, while 
UVA can reach capillary blood.  Therefore, clinical relevance of photochemical activation by 
UVB is considered less important than activation by UVA for systemic drugs.  However, UVB 
irradiation is relevant for topical formulations applied to light-exposed tissues. 
 
The selection and monitoring of appropriate light sources (spectral distribution, irradiance, and 
dose) and the procedures used should be clearly described in the study methodology (e.g., OECD 
TG 432 (Ref. 6)). 
 
B.  Photoreactivity Tests Using Chemical Assays (3.2) 
   
If a drug developer chooses to assess photoreactivity, the assay should be qualified using 
pharmaceutical agents under appropriate conditions to demonstrate assay sensitivity.  One such 
assay is an ROS assay (e.g., Ref. 7).  Data suggest that this assay has high sensitivity for 
predicting direct in vivo phototoxicants.  However, this assay has a low specificity, generating a 
high percentage of false positive results.  A negative result in this assay, conducted under the 
appropriate conditions, would indicate a very low probability of phototoxicity, provided a test 
concentration of 200 M can be achieved, whereas a positive result (at any concentration) would 
only be a flag for follow-up assessment. 
 
C.  Phototoxicity Tests Using In Vitro Assays (3.3) 
 
A number of in vitro assays have been developed for assessing the phototoxicity potential of 
chemicals.  Some of these assays have not been qualified for use with pharmaceuticals.  Some 
assays involve testing compounds that are dissolved in the culture medium, and such methods are 
often appropriate for the active ingredient or excipients in drug products, depending on their 
solubility.  Other assays involve direct application to the surface of a tissue preparation and can be 
appropriate for testing entire formulations intended to be administered topically. 
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  Contains Nonbinding Recommendations 
 
 
The most widely used in vitro assay for phototoxicity is the 3T3 neutral red uptake phototoxicity 
test (3T3 NRU-PT) for which an Organization for Economic Co-operation and Development 
(OECD) guideline (Ref. 6) is available.  This is currently considered the most appropriate in vitro 
screen for soluble compounds. 
 
Although the formal European Centre for the Validation of Alternative Methods (ECVAM) 
validation exercise conducted on this assay indicated a sensitivity of 93% and a specificity of 84%, 
experience within the pharmaceutical industry suggests a much lower specificity.  The original 
OECD protocol was not validated for pharmaceuticals specifically.  Thus, some modifications to 
the original OECD protocol have been proposed to address the low specificity observed with drug 
substances (see Note 4).  These proposed changes are appropriate for the testing of 
pharmaceuticals.  The sensitivity of the 3T3 NRU-PT is high, and if a compound is negative in 
this assay, it would have a very low probability of being phototoxic in humans.  However, a 
positive result in the 3T3 NRU-PT should not be regarded as indicative of a likely clinical 
phototoxic risk, but rather a flag for follow-up assessment.  
 
The BALB/c 3T3 cell line is sensitive to UVB, and the initially recommended irradiation 
conditions (Ref. 6) involve the use of filters to attenuate wavelengths below 320 nm.  However, 
depending on the light source and filters used, the ratio of UVB to UVA can be adjusted such that 
it is possible to assess UVB-induced phototoxicity in this test.  UVB-induced phototoxicity is 
rarely a problem for pharmaceuticals with systemic exposure because UVB minimally penetrates 
beyond the epidermis.  However, UVB-induced phototoxicity is more relevant for topical 
products.  For components of topically applied products that absorb predominately in the UVB 
range, and if in vitro assessment is desired, the use of the 3T3 NRU-PT with modified irradiation 
conditions (see above) can be considered.  Alternatively, in vitro skin models, which better 
tolerate UVB, could be considered. 
 
Reconstructed human skin models, with the presence of a stratum corneum, permit testing of 
various types of topically applied materials ranging from neat chemicals to final clinical 
formulations.  The assays developed with reconstructed human skin to date measure cell viability 
with and without irradiation. These assays appear to be capable of detecting known human acute 
dermal phototoxicants.  However, the sensitivity of some assays can be less than that of human 
skin in vivo, wherein the lowest concentration eliciting a positive response can be higher than in 
human skin in vivo.  Consequently, it is important to understand the sensitivity of any assay 
selected and, if appropriate and feasible, to adjust the assay conditions accordingly (e.g., testing 
higher strength formulations, increasing exposure time). 
 
There are no in vitro models that specifically assess ocular phototoxicity, regardless of the route of 
administration.  While negative results in the 3T3 NRU-PT or a reconstructed human skin assay 
might suggest a low risk, the predictive value of these assays for ocular phototoxicity is unknown. 
 
D.  Photosafety Tests Using In Vivo Assays and Systemic Administration (3.4) 
 
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Phototoxicity testing for systemically administered compounds has been conducted in a variety of 
species, including guinea pig, mouse, and rat.  No standardized study design has been established, 
and thus the following factors might be considered as best practices. 
 
For species selection, irradiation sensitivity (i.e., minimal erythema dose), heat tolerance, and 
performance of reference substances should be considered.  Models with both pigmented and 
non-pigmented animals are available.  Although non-pigmented skin tends to be more sensitive 
than pigmented skin for detecting phototoxicity, pigmented skin should be considered for APIs 
that bind significantly to melanin (see section II.B(2.2)) if appropriate exposures in target tissues 
cannot be ensured otherwise. 
 
If an in vivo phototoxicity study is conducted, it is desirable to have some information about the 
pharmacokinetic profile of the compound before designing the study.  This is to ensure that 
irradiation of the animals is conducted at the approximate time at which the maximum 
concentration of drug occurs after drug administration (T  and to assist in the selection of an 
max),
appropriate study duration in relation to the intended clinical exposure.  Relevant 
pharmacokinetic data, if not already available, should be collected as part of the in vivo 
phototoxicity study. 
 
Although phototoxicity is typically an acute reaction, the duration of an in vivo assay should be 
carefully considered.  Accumulation of compound in relevant light-exposed tissues after repeated 
administration might lead to an increased phototoxic response.  Similarly, repeated irradiation 
after each dose might also lead to an increased phototoxic response due to the accumulation of 
damage.  Generally, studies of a single day or up to a few days duration of dosing are 
appropriate, using the clinical route of administration, if feasible.  Single or repeated daily 
irradiations after dosing (around T ) can be used. 
max
 
Dose selection for in vivo nonclinical phototoxicity testing of systemic drugs should support a 
meaningful human risk assessment.  For such studies, a maximum dose level that complies with 
the recommendations for general toxicity studies in ICH M3(R2), section I.E(1.5), is considered 
appropriate.  If a negative result is obtained at the maximum dose, testing of lower doses is 
usually not warranted.  However, if a positive result is anticipated, additional dose groups can 
support a NOAEL-based (no observable adverse effect level-based) risk assessment, typically 
considering the maximum concentration of drug after administration (C ) comparisons. Vehicle 
max
and non-irradiated controls can help identify compound-related phototoxicity and distinguish 
irradiation-induced from non-irradiation-induced adverse reactions.  If the maximum systemic 
exposure achieved in animals is lower than clinical exposure, the reliability of a negative result in 
predicting human risk is questionable.  
 
The most sensitive early signs of compound-induced phototoxicity are usually erythema followed 
by edema at a normally sub-erythemogenic irradiation dose.  The type of response might vary 
with the compound.  Any identified phototoxicity reaction should be evaluated regarding dose 
and time dependency, and if possible, the no observable adverse effect level (NOAEL) should be 
established.  The hazard identification might be further supported by additional endpoints (e.g., 
early inflammatory markers in skin or lymph node reactions indicative of acute irritation). 
 
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If a phototoxicity study is conducted in animals for a systemic drug that absorbs light above 400 
nm, phototoxicity of the retina should be assessed using a detailed histopathological evaluation. 
For compounds that only absorb light below 400 nm, retinal assessment is usually not warranted 
because such wavelengths do not reach the retina of the adult human eye due to limited penetration 
of the cornea, lens, and vitreous body. 
 
Adequate performance of in vivo phototoxicity assays, which are not formally validated, should be 
demonstrated using suitable reference compounds, including pharmaceuticals.  Compounds that 
are phototoxic in humans and that represent different chemical classes and mechanisms of 
phototoxicity should be included to establish adequacy of the assays.  For retinal phototoxicity, a 
reference compound with a light absorption profile within the visible light range (i.e., above 400 
nm) is recommended.  The concurrent use of a positive control compound might not be warranted 
if an in vivo assay has been formally validated or has reached general acceptance and is established 
in the testing facility. 
 
Testing for photoallergy is not recommended for compounds that are administered systemically.  
Photoallergy reactions in humans following systemic administration are rare, and there are no 
established nonclinical photoallergy assays for systemically administered compounds. 
 
E.  Photosafety Tests Using In Vivo Assays and Dermal Administration (3.5) 
 
The main recommendations provided for investigating the systemic route of administration also 
apply to dermal administration, including those for species selection, study duration, and 
irradiation conditions.  For dermal drug products in general, the clinical formulation should be 
tested.  The intended clinical conditions of administration should be used to the extent possible.  
Irradiation of the exposed area should take place at a specified time after application, and the 
interval between application and irradiation should be justified based on the specific properties of 
the formulation to be tested.  Signs of phototoxicity should be assessed based on relevant 
endpoints (see section III.D(3.4)).  The sensitivity of the assay should be demonstrated using 
appropriate reference compounds.  Assessment of systemic drug levels is generally not warranted 
in dermal phototoxicity studies. 
 
For dermal drug products, contact photoallergy has often been assessed in a nonclinical study 
along with acute phototoxicity (photoirritation).  However, no formal validation of such assays 
has been performed.  Although the acute photoirritation observed in these studies is considered 
relevant to humans, the predictivity of these studies for human photoallergy is unknown.  For 
regulatory purposes, such nonclinical photoallergy testing is generally not recommended. 
 
 
IV.  CLINICAL PHOTOSAFETY ASSESSMENT (4) 
 
There are various options for collecting human data, if warranted, ranging from standard reporting 
of adverse events in clinical studies to a dedicated clinical photosafety trial.  The precise strategy 
is determined on a case-by-case basis. 
 
 
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V.  ASSESSMENT STRATEGIES (5) 
 
The choice of the photosafety assessment strategy is up to the drug developer.  ICH M3(R2) 
suggests that an initial assessment of the phototoxicity potential based on photochemical 
properties and pharmacological/chemical class be undertaken before outpatient studies. 
Characterization of the UV-visible absorption spectrum is recommended as the initial assessment 
because it can obviate any further photosafety evaluation.  In addition, the distribution to skin and 
eye can be evaluated to inform further on the human risk and the recommendations for further 
testing.  Then, if appropriate, an experimental evaluation of phototoxicity potential (in vitro, in 
vivo, or clinical) should be undertaken before exposure of large numbers of subjects (phase 3). 
 
Figure 1 provides an outline of possible phototoxicity assessment strategies. The figure is based on 
the strategies outlined in this section of this guidance.  The strategies are flexible.  Depending on 
the particular situation, some portions of the assessment are optional and might not be conducted. 
 
A.  Recommendations for Pharmaceuticals Given via Systemic Routes (5.1) 
 
1.  Assessment of Phototoxicity Potential (5.1.1) 
 
If the substance does not have an MEC greater than 1000 L mol-1 cm-1 (between 290 and 700 nm), 
no photosafety testing is recommended and no direct phototoxicity is anticipated in humans.  
However, it should be noted that phototoxicity by indirect mechanisms (e.g., pseudoporphyria or 
porphyria), although rare, could still occur.  For compounds with MEC values of 1000 L mol-1 
cm-1 or higher, if the drug developer chooses to conduct a test for photoreactivity, a negative result 
could support a decision that no further photosafety assessment is warranted (see section 
III.B(3.2)).  Otherwise, nonclinical and/or clinical photosafety assessment of the substance 
should be conducted.  Available data on the phototoxicity of chemical class-related compounds 
should be evaluated because this could inform on the approach to be taken. 
 
2.  Evaluation of Phototoxicity (5.1.2) 
 
To reduce the use of animals in accordance with the 3R principles, a validated in vitro method 
should generally be considered before conducting animal testing (see, for example, Directive 
2010/63/EU).  If the drug developer chooses an in vitro approach, the 3T3 NRU-PT is currently 
the most widely used assay and in many cases could be considered as an initial test for 
phototoxicity.  The high sensitivity of the 3T3 NRU-PT results in good negative predictivity and 
the negative results are generally accepted as sufficient evidence that a substance is not phototoxic.  
In such cases no further testing is recommended and no direct phototoxicity is anticipated in 
humans.  
 
In some situations (e.g., poorly soluble compounds), an initial assessment of phototoxicity in an in 
vitro assay might not be appropriate.  In this case, an assessment in animals or in humans could be 
considered.  Alternatively, if drug distribution data are available, they could, on a case-by-case 
basis, support a decision that no further photosafety assessment is warranted (see section 
II.B(2.2)). 
 
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If an in vitro phototoxicity assay gives a positive result, a phototoxicity study in animals could be 
conducted to assess whether the potential phototoxicity identified in vitro correlates with a 
response in vivo.  Alternatively, drug distribution data could, on a case-by-case basis, support a 
position that the risk of phototoxicity in vivo is very low and that no further photosafety 
assessment is warranted (see section II.B(2.2)).  As another option, the photosafety risk could be 
assessed in the clinical setting or managed by the use of light-protective measures.  A negative 
result in an appropriately conducted phototoxicity study either in animals or humans supersedes a 
positive in vitro result.  In such cases, no further testing is recommended and no direct 
phototoxicity is anticipated in humans. 
   
A positive result in an in vivo animal study can, in certain circumstances, be mitigated using a 
NOAEL-based risk assessment, typically considering C  comparisons.  Otherwise, a clinical 
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assessment is warranted.  In all cases, a robust clinical phototoxicity assessment indicating no 
concern supersedes any positive nonclinical results.  
 
A positive result in an in vitro phototoxicity test would not be negated by a negative result in a 
subsequently conducted chemical photoreactivity assay (e.g., an ROS assay). 
 
In cases where an animal or clinical phototoxicity study has already been conducted, there is no 
reason to subsequently conduct either a chemical photoreactivity or an in vitro phototoxicity assay. 
 
B.  Recommendations for Pharmaceuticals Given via Dermal Routes (5.2) 
 
1.  Assessment of Phototoxicity Potential (5.2.1) 
 
If the active substance and excipients do not have MEC values greater than 1000 L mol-1 cm-1 
(between 290 and 700 nm), no further photosafety testing is recommended and no phototoxicity is 
anticipated in humans.  For compounds with MEC values of 1000 L mol-1 cm-1 or higher, negative 
photoreactivity test results (e.g., an ROS assay) can support a decision that no further photosafety 
assessment is warranted (see Note 5 for exception).  If further assessment is warranted, available 
data on the phototoxicity of chemical class-related compounds should be evaluated, because this 
could inform on the approach to be taken. 
 
Tissue distribution is not a consideration for the phototoxicity of dermal products.  Dermal 
products are administered directly to the skin and hence, unless they are applied to areas not 
usually exposed to light, are assumed to be present in light-exposed tissues. 
 
2.  Experimental Evaluation of Phototoxicity and Photoallergy (5.2.2) 
 
The 3T3 NRU-PT can be used to assess individually the phototoxicity potential of the API and any 
new excipient(s), provided that appropriate testing conditions can be achieved (e.g., if test 
concentrations are not limited by poor solubility, relevant UVB dose can be applied).  In cases 
where no phototoxic component has been identified in vitro, the overall phototoxicity potential of 
the clinical formulation can be regarded as low.  
 
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Some properties of the clinical formulation that could influence the potential phototoxic response 
(e.g., penetration into skin, intracellular uptake) cannot be evaluated using the 3T3 NRU-PT alone. 
Therefore, confirmation of the overall negative result in an evaluation using the clinical 
formulation and/or monitoring during clinical trials can still be warranted. 
 
Reconstructed human skin models can be used to assess the phototoxicity potential of clinical 
formulations.  Under adequate test conditions (see section III.C(3.3)), a negative result in a 
reconstructed human skin assay indicates that the direct phototoxicity potential of the formulation 
can be regarded as low.  In this case, generally no further phototoxicity testing is recommended 
(see Note 5 for exception). 
 
If an appropriate in vitro assay is not available, the initial test could be an in vivo phototoxicity test 
on the clinical formulation.  A negative result in an appropriately conducted in vivo animal 
phototoxicity study would be sufficient evidence that the formulation is not directly phototoxic 
and no further phototoxicity testing is recommended (see Note 5 for exception).  Alternatively, the 
phototoxicity potential can be assessed in the clinical setting. 
 
For dermal products where the API or any new excipient has an MEC value greater than 1000 L 
mol-1 cm-1 at any wavelength between 290 and 700 nm, a photoallergy assessment is generally 
warranted in addition to phototoxicity testing.  Because the predictivity of nonclinical 
photoallergy tests is unknown, this would typically be a clinical assessment using the 
to-be-marketed formulation and conducted during phase 3.   
 
Photosafety evaluation of the clinical formulation delivered via dermal patches can follow the 
above described principles for clinical dermal formulations.  For transdermal patches, the 
principles for both dermal and systemic drugs should be applied.  In addition, the intended 
clinical use (e.g., skin area recommended for use, duration of application) and the properties of the 
patch matrix (e.g., being opaque to UV and visible light) should be considered for the overall risk 
assessment.
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Figure 1:  Outline of Possible Phototoxicity Assessment Strategies for Pharmaceuticals 
Given via Systemic and Dermal Routes 
 
 
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VI.  ENDNOTES (6) 
 
Note 1:  For compounds that absorb at relevant wavelengths, have an MEC value greater than 
1000 L mol-1 cm-1, and are given via ocular routes (e.g., eye drops, intraocular injections), an 
evaluation of the phototoxicity potential should be undertaken in accordance with the general 
principles of phototoxicity assessment.  Biodistribution of drug in the eye and optical properties 
of the eye should also be considered.  Any available information on the compound or chemical 
class-related compounds should be considered in the overall assessment. 
 
Compounds that only absorb light at wavelengths below 400 nm and are to be administered as 
intraocular injections behind the lens (e.g., in the vitreous) are of low concern for retinal 
phototoxicity, because only light of wavelengths greater than 400 nm reaches the back of the adult 
eye.  However, the lens in children of less than approximately 10 years of age is not completely 
protective against wavelengths below 400 nm. 
 
Note 2:  Testing for photogenotoxicity is not recommended as a part of the standard photosafety 
testing program.  In the past, some regional guidelines (e.g., CPMP/SWP/398/01) have 
recommended that photogenotoxicity testing be conducted preferentially using a 
photoclastogenicity assay (chromosomal aberration or micronucleus test) in mammalian cells in 
vitro.  However, experience with these models since the CPMP/SWP guideline was issued has 
indicated that these tests are substantially oversensitive and even incidences of 
pseudo-photoclastogenicity have been reported (Ref. 8).  Furthermore, the interpretation of 
photogenotoxicity data regarding its meaning for clinically relevant enhancement of UV-mediated 
skin cancer is unclear. 
   
Note 3:  Standardized conditions for determination of MECs are critical.  Selection of an 
adequate solvent is driven by both analytical requirements (e.g., dissolving power, UV-visible 
light transparency) and physiological relevance (e.g., pH 7.4-buffered aqueous conditions).  
Methanol is recommended as a preferred solvent and was used to support the MEC threshold of 
1000 L mol-1 cm-1 (Ref. 3).  When measuring UV-visible light spectra, potential limitations (e.g., 
artifacts due to high concentrations or low solubility, including slow precipitation) should be 
considered.  If the chromophore of the molecule appears to be pH-sensitive (e.g., phenolic 
structure, aromatic amines, carboxylic acids), an additional spectrum obtained under aqueous, pH 
7.4-buffered conditions could add valuable information regarding differences in the shape of the 
absorption spectrum and in the MECs.  If significant differences are seen between measurements 
obtained in methanol versus pH-adjusted conditions, the MEC threshold of 1000 L mol-1 cm-1 
cannot be used to obviate further photosafety assessment. 
 
Note 4:  A survey of pharmaceutical companies indicated that the 3T3 NRU-PT, as described in 
OECD TG 432, generates a high percentage of positive results (approximately 50%), the majority 
of which do not correlate with phototoxicity responses in animals or humans (Ref. 9).  Following 
a retrospective review of data for pharmaceuticals, a reduction of the maximum test concentration 
from 1000 to 100 g/mL appears justified (Ref. 10).  Compounds without any significant 
cytotoxicity (under irradiation) up to this limit can be considered as being devoid of relevant 
phototoxicity.  In addition, the category named probable phototoxicity per OECD TG 432 (i.e., 
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photoirritation factor (PIF) values between 2 and 5 or mean photo effect (MPE) values between 
0.10 and 0.15) is of questionable toxicological relevance for systemic drugs.  Compounds in this 
category generally do not warrant further photosafety evaluations.  For compounds with a PIF 
value between 2 and 5, and for which it is not possible to determine an IC  in the absence of 
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irradiation, it is important to check that the compound is not classified as positive using the MPE 
calculation (i.e., that the MPE is less than 0.15). 
 
Systemic drugs that are positive in the 3T3 NRU-PT only at in vitro concentrations that are many 
times higher than drug concentrations likely to be achieved in light-exposed tissues in humans, 
can, on a case-by-case basis and in consultation with regulatory authorities, be considered to be 
low risk for phototoxicity in humans, without follow-up in vivo testing. 
 
Note 5:  In the United States, for products applied dermally, a dedicated clinical trial for 
phototoxicity (photoirritation) on the to-be-marketed formulation (API plus all excipients) can be 
warranted in support of product approval.
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VII.  GLOSSARY (7) 
 
3T3 NRU-PT:  In vitro 3T3 neutral red uptake phototoxicity test. 
 
Assessment:  In the context of this document, an assessment is an evaluation of all available 
information and does not always mean an additional test is conducted. 
  
Chromophore:  The substructure of a molecule that absorbs visible or ultraviolet light. 
 
Dermal Drugs:  Products applied topically to the skin. 
 
Direct Phototoxicity:  Phototoxicity induced by absorption of light by the drug or excipient. 
 
Indirect Phototoxicity:  Phototoxicity due to cellular, biochemical, or physiological alterations 
caused by the drug or excipient, but not related to photochemical reactivity of the drug or excipient 
(e.g., perturbation of heme homeostasis). 
 
Irradiance:  The intensity of UV or visible light incident on a surface, measured in W/m2 or 
mW/cm2. 
 
Irradiation:  The process by which an object/subject is exposed to UV or visible radiation. 
 
MEC:  Molar extinction coefficient (also called molar absorptivity) reflects the efficiency with 
which a molecule can absorb a photon at a particular wavelength (typically expressed as L mol-1 
cm-1) and is influenced by several factors, such as solvent. 
  
MPE:  The mean photo effect is calculated for results of the 3T3 NRU-PT.  The MPE is based 
on comparison of the complete concentration response curves (see OECD TG 432). 
  
NOAEL:  No observed adverse effect level. 
 
OECD TG:  Organisation for Economic Co-operation and Development, Test Guideline. 
 
Outpatient Study:  A clinical study in which patients are not restricted to a clinical site. 
 
Photoproducts:  New compounds/structures formed as a result of a photochemical reaction. 
 
Photoreactivity:  The property of chemicals to react with another molecule as a consequence of 
absorption of photons. 
  
PIF:  Photoirritation factor is calculated for results of the 3T3 NRU-PT by comparing the IC  
50
values obtained with and without irradiation. 
 
ROS:  Reactive oxygen species, including superoxide anion and singlet oxygen. 
 
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Systemic Drugs:  Products administered by a route that is intended to produce systemic 
exposure. 
 
UVA:  Ultraviolet light A (wavelengths between 320 and 400 nm).  
 
UVB:  Ultraviolet light B (wavelengths between 280 and 320 nm; as a part of sunlight 
wavelengths between 290 and 320 nm).
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VIII.  REFERENCES 
 
1.  ICH guidance, M3(R2) Nonclinical Safety Studies for the Conduct of Human Clinical Trials 
and Marketing Authorization for Pharmaceuticals (Revision 1), June 2009. 
 
2.  ICH guidance, S9 Nonclinical Evaluation for Anticancer Pharmaceuticals, March 2010. 
 
3.  Bauer D, Averett LA, De Smedt A, Kleinman MH, Muster W, Pettersen BA, Robles C. 
Standardized UV-vis spectra as the foundation for a threshold-based, integrated photosafety 
evaluation. Regul Toxicol Pharmacol 2013, in press. 
 
4.  ICH guidance, Q1B Photostability Testing of New Drug Substances and Products, November 
1996. 
 
5.  Technical report, CIE-85-1989: Solar Spectral Irradiance; January 1989. 
 
6.  OECD (2004), Test No. 432: In Vitro 3T3 NRU Phototoxicity Test, OECD Guidelines for 
the Testing of Chemicals, Section 4, OECD Publishing. 
 
7.  Onoue S, Igarashi N, Yamada S, Tsuda Y. High-throughput reactive oxygen species (ROS) 
assay: An enabling technology for screening the phototoxic potential of pharmaceutical 
substances. J Pharm Biomed Anal 2008; 46(1): 187193.  
 
8.  Lynch AM, Robinson SA, Wilcox P, Smith MD, Kleinman M, Jiang K, Rees RW. 
Cycloheximide and disulfoton are positive in the photoclastogenicity assay but do not absorb 
UV irradiation: Another example of pseudophotoclastogenicity? Mutagenesis 2008; 23(2): 
111118. 
 
9.  Lynch, AM, Wilcox, P. Review of the performance of the 3T3 NRU in vitro phototoxicity 
assay in the pharmaceutical industry. Exp Toxicol Pathol 2011; 63(3): 209214. 
 
10. Ceridono M, et al. Workshop Report: The 3T3 neutral red uptake phototoxicity test: Practical 
experience and implications for phototoxicity testing  the report of an ECVAMEFPIA 
workshop. Regul Toxicol Pharmacol 2012; 63(3): 480488. 
 
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