Abstract:
With the increasing diversity of drug forms, the number of drugs involved in photosafety evaluation has also increased, and the frequency of photosafety being mentioned and discussed has also increased. In fact, many drugs can cause acute to chronic, reversible to irreversible damage to the human body under light conditions. According to the different characteristics of the damage, photosafety evaluation is usually divided into four categories: phototoxicity, photoallergic, photogenotoxic, and photocarcinogenic evaluation. Among them, phototoxicity is the most common. This article focuses on the relevant guidelines, development strategies, and common problems of photosafety evaluation.
Keywords: photosafety, phototoxicity, photoallergic, photogenotoxic, photocarcinogenic, drug
1. Common Phototoxic Drugs
Some drugs, after being taken, can cause allergies in the human body under light stimulation. These drugs are called photosensitive drugs. The main manifestations of drug-induced photosensitivity reactions are: erythema and edema of the skin exposed to light, accompanied by itching, burning pain, or pigmentation. In severe cases, there may be blisters. After the blisters rupture, ulcers or erosions may form.
Common phototoxic drugs include:
- Quinolone antibiotics, such as sparfloxacin, ofloxacin, ciprofloxacin, and lomefloxacin.
- Sulfonamide antibiotics, such as tetracyclines and chloramphenicol.
- Antipsychotics, such as chlorpromazine.
- Diuretics, such as hydrochlorothiazide, furosemide, and spironolactone.
- Hypoglycemic drugs, such as gliclazide and glipizide.
- Anticancer drugs, such as vincristine and methotrexate.
2. Mechanism of Phototoxicity
In the article “Research Progress and Evaluation Strategy of Drug Photosafety Evaluation”, Honggang Tu et al. described four mechanisms of phototoxicity. After some drugs are exposed to light, they are converted from a stable state to an excited state, which can then damage the body. First, the directly toxic photoproducts cause cell damage; second, the excited singlet oxygen is produced, which oxidizes biological macromolecules, leading to damage to the cell membrane or DNA; third, superoxide, peroxide, and hydroxyl radicals are produced, which damage the cell membrane or DNA; fourth, the drug group is changed and covalently bonded to biological macromolecules to form an antigen, which then produces an immune response.
3. How to Conduct Phototoxicity Studies of Drugs
3.1 Guidelines for Phototoxicity Studies
In January 2015, the FDA issued the Photosafety Testing (S10) Guidance for Drugs. The purpose of this guidance is to recommend international standards for drug photosafety testing. The interpretation of this document can be combined with the content of Section 14 of ICH M3(R2) on photosafety evaluation.
ICH S10: Photosafety evaluation of pharmaceuticals.
When conducting photosafety testing, the evaluation should include the reactions of phototoxicity, photoallergic, photogenotoxic, and photocarcinogenic. Photoallergic reaction is an immune response caused by the photoproduct produced by the drug after photochemical reaction. In the evaluation, non-animal methods or clinical data will also be used to evaluate photosafety as much as possible. However, the guidance principles do not apply to peptides, proteins, antibody conjugates, or oligonucleotide drugs, as well as products with new safety concerns for the API or excipients (unless the dosage form of the product has been changed).
3.2 Strategy for Phototoxicity Evaluation
Both antitumor and non-antitumor drugs follow ICH-S10 for phototoxicity evaluation.
Raw materials that do not have ultraviolet absorption characteristics generally do not need to be tested. However, how to judge “no ultraviolet absorption characteristics” is a problem. We know that the frequency of ultraviolet light in the spectrum can be divided into UVA (400nm～320nm), UVB (320nm～280nm), UVC (280nm～100nm), and EUV (100nm～10nm). The main ultraviolet light source in nature is the sun. When sunlight passes through the atmosphere, ultraviolet light with a wavelength shorter than 290nm will be absorbed by the ozone in the atmosphere. Therefore, the wavelength of natural sunlight is generally between 290 and 700nm, and the ultraviolet light that we are exposed to daily is mainly in the wavelength range of 290nm～400nm.
According to ICH regulations, ultraviolet absorption spectrum can be used to:
- Determine whether the substance has a maximum absorption at 290nm～400nm. Substances that do not have obvious absorption in this wavelength range generally do not have ultraviolet absorption characteristics.
- If the substance has weak absorption at 290nm～400nm, calculate the molar extinction coefficient (MEC) in the range of 290nm～400nm.
At the initial IND, it is necessary to evaluate the potential possibility of phototoxicity through photochemical properties at least. To investigate whether the compound can absorb photons in the wavelength range of 290~700nm. ICH S10 points out that when the molar extinction coefficient (MEC) of a compound in the wavelength range of 290~700nm is not higher than 1000L/mol/cm, it is considered that the compound does not have enough photoreactivity to produce direct phototoxicity. If it is higher than 1000L/mol/cm, the non-clinical tissue distribution test results can be used to investigate whether the compound is distributed in relevant tissues such as skin and choroid. If so, or if non-clinical tissue distribution studies were not conducted before the IND application, relevant photoprotection measures can be added to the clinical research plan to protect the subjects.
OECD TG 432 also mentions that if the molar extinction coefficient (MEC) is less than 1000L/mol/cm (measured in methanol), the chemical is unlikely to undergo photoreaction, and these chemicals may not require phototoxicity testing.
First, the risk is identified according to the MEC value to determine whether the drug needs to be tested for phototoxicity. If necessary, a study based on an in vitro test system can be conducted first. The 3T3 neutral red uptake phototoxicity test method (3T3 NRU-PT) is generally preferred. If the result is negative, no further study is needed. If positive, it is necessary to combine tissue distribution data to conduct in vivo phototoxicity studies. If the in vivo phototoxicity result is positive, a photoallergic test should be conducted. In this regard, EMA (EMA, 2011) and ICH (S10) both believe that systemically administered drugs do。
3.3 Common Methods for Evaluating Photosafety
Phototoxicity evaluation methods include 3T3 NRU-PT, red blood cell phototoxicity method, human skin model, Photo-RBC test, human keratinocyte method, and human lymphocyte method. Among them, 3T3 NRU-PT has been included in the OECD Chemical Toxicity Guidelines and is the most widely used. The in vivo phototoxicity evaluation method can select species such as pigs, rabbits, guinea pigs, rats or mice according to whether the drug is administered systemically or applied topically to the skin, and rationally design the administration frequency, irradiation time point, irradiation dose, irradiance, irradiation duration, and light source.
Photoallergic reaction is a delayed type hypersensitivity reaction mediated by light and based on cellular immune response. After absorbing light energy, the drug becomes activated and binds to proteins in the skin in a semi-antigen form to form a drug-protein complex (complete antigen), which is transmitted to immune active cells by Langerhans cells in the epidermis, causing an allergic reaction. Photoallergic reaction belongs to type Ⅳ delayed hypersensitivity reaction, which has a relatively long occurrence time and a certain latent period. Photoallergic reaction is mainly based on guinea pig model, and mouse ear swelling and human serum protein binding test are also used.
Photogenetic toxicity refers to the genetic toxicity caused by the activation of compounds under light conditions. The main evaluation methods currently include Photo-Ames test, Photo-HPRT test, Photo-MLA test, and Photo-comet test.
Photocarcinogenicity refers to the direct (photochemical carcinogenesis) or indirect increase in the occurrence of skin tumors related to ultraviolet radiation. The only model that complies with GLP regulations is the SKH1 (hr/hr) nude mouse model, but its human predictive value is unknown.
4. Specific Technical Requirements
Phototoxicity evaluation needs to distinguish whether the drug is administered systemically or topically.
For systemically administered drugs, the research strategy in Section 3.2 can be followed. First, the MEC result is used to determine whether a specific test is needed. Then, according to the 3R principle, an in vitro method is preferred. 3T3 NRU-PT is currently the most widely used test and is usually used for preliminary screening of phototoxicity.
In some cases (such as compounds with poor solubility), it may not be appropriate to use in vitro tests for preliminary evaluation of phototoxicity. At this time, animal in vivo or human test evaluation should be considered. Or according to the principle of specific analysis of specific problems, if there is drug distribution data, it can support the decision of no need for further photosafety evaluation.
If the in vitro phototoxicity test result is positive, an animal in vivo phototoxicity test is needed to evaluate the correlation between the potential phototoxicity determined by the in vitro test and the in vivo test result. Or based on the principle of specific analysis of specific problems, if the drug distribution data indicates that its in vivo phototoxicity risk is very low, no further photosafety evaluation is needed. Another option is to evaluate the photosafety risk during clinical trials, or take photoprotection measures during clinical trials. The reference value of a negative result of a suitable animal in vivo or human phototoxicity test is greater than that of a positive in vitro test, and in this case, it is not recommended to conduct further tests, and it can be predicted that there is no direct phototoxicity to the human body.
In some cases, the risk level indicated by the positive result of the in vivo animal test can be reduced in the risk assessment based on NOAEL, and the comparison with Cmax is usually considered. Otherwise, clinical evaluation is needed. In all cases, if a reliable clinical phototoxicity evaluation