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All images in this presentation are the property of Jane Hanrahan unless otherwise referenced.
References1. http://medlib.med.utah.edu/kw/derm/pages/meet_2.htm
accessed 16/7/062. D. E. Moore, Drug Safety, 25, 345-372 (2002)
After completion of these lectures you should be able
to:
1. Describe the effects that the different classes of UV radiation have on the skin at the earths surface.
2. Be able to describe structural features common to drugs that result in photosensitivity reactions.
3. Explain the two different mechanisms of photodynamic action and the role in which biological molecules paly in photodynamic action.
4. Understand the difference between photoallergy and phototoxicity.5. Describe how certain NSAIDS cause photosensitivity reactions and understand how this is
related to their structure.6. Explain the cellular defence mechanisms that biological systems use to protect themselves
from UV radiation.7. Describe the ideal properties of sunscreens and explain the mechanisms by which various
sunscreens are effective.8. Understand how phototherapy is effective in treating neo-natal jaundice.9. Be able to describe the mechanism through which psoralens in conjunction with UVA
radiation are effective in treating skin diseases such as psoriasis.10. Describe the ideal properties of PDT drugs and their use in the treatment of various cancers
and skin diseases. Understand the mechanism by which these drugs act.11. Be able to explain the role of oxygen and radiation sensitisers in the radiotherapy of
tumours.
Radiation Effects
A/Prof Jane [email protected]
Objectives
Describe the effects of low-energy (non-ionising) radiation on the human body.Sunburn
Understand how sunscreens work
Drug induced photosensitivity - phototoxicity and photoallergy
Suntan Exposure restricted to minimal erythemal dose
(MED) Thickening of epidermis Darkening of existing pigmentation New pigment formation
Melanin Proliferative changes in epidermis Suppression of T-lymphocytes
Sunburn
Stage 1 Immediate faint erythema
Occurs during exposure - disappears quickly
Stage 2 Delayed erythemal exposure
(2-4 h, peaks at 14-20 h, persists 24-48 h)
Sunburn
Stage 3 Dead cells form a desquamating layer
Desquamation begins 72-96 h
Stage 4 Increase in DOPA +vemelanocytes
Stage 5 Repeated sunburn skin cancer
Sunburn
Markers Melanin tanning Langerhans cells
(macrophage - like cells of the epidermis) Ornithinedecarboxylase activity
Action Spectrum for induction of
erythema
Action spectrum (approximate) for the induction of erythema in human skin, and average intensity at the earth’s surface. The figure also depicts the ranges of various UV regions. D. E. Moore, Drug Safety, 25, 345-372 (2002)
Skin Layers and UV Light Penetration
(200-280nm) blocked by ozone layerProduced by some artificial light sourcesMay penetrate top layers of skin(280-320nm) responsible for sunburnContributes little to tanningCauses squamous cell carcinoma and Leathery skin(320-400nm) penetrates deep into theDermis.Causes wrinkles, blotches & age-spotsStimulate melanin to tanContributes to sunburn
200 250 300
UVC
UVB
UVA
http://medlib.med.utah.edu/kw/derm/pages/meet_2.htm accessed 16/7/06
Penetration and effects of
UV radiation
Absorption spectrum of DNA &
protein
Absorption spectra of DNA (calf thymus) and protein (bovine serum albumin at equal concentrations (20 µg/ml). D. E. Moore, Drug Safety, 25, 345-372 (2002)
UV chromophores in the skin
D. E. Moore, Drug Safety, 25, 345-372 (2002)
Clinical Phototoxic Responses
Strong delayed erythema Onset 8-24 hours lasts 2-4 days May be darker than normal sunburn egpsoralens
More rapid, transient erythema Immediate onset, lasts 1-2 days No oedema, localised burning and itching Eg coal tar derivatives, anthracene, acridine
Rapid transient wheals and flares Eghaematoporphyrin (cancer treatment)
ADRAC (Australia)Reports to December 1994
Diuretic Agents Moduretic (Hydrochlorothiazide + Amiloride) 320 Dyazide (Hydrochlorothiazide + Triamterene) 156 Furosemide (furosemide) 148 Hydrochlorothiazide 72 Chlorothiazide 27
Other Combination Formulations Bactrim/Septrin 204 (sulfamethoxazole + trimethoprim) Tetracycline + Nystatin 66
Attempted differentiation
between phototoxicity and
photoallergyObservation Result in
PhototoxicityResult in Photoallergy
Reaction to first exposure Present Absent
Latency between exposure & response Variable May occur
Gross reactions to structurally related compounds
Absent Varied
Clinical changes Like sunburn Varied
Flares at previously involved sites Never Possible
Development of persistent light reaction
Never Rarely
Incidence for a given compound Very high Usually low
Concentration of drug required for reaction
High Low
Action spectrum & absorption spectrum
Normally similar
Action spectrum at longer
Results of photo patch Immediate Delayed
Examples of photosensitising drugs
Sulfonamides Furocoumarins
Chlorodiazepoxide
Methyldopa
Protriptylline
Examples of photosensitising drugs
Norethisterone Nalidixic acid
Tetracyclines
Chlorpromazine Thiazides
Common structural features of
Photosensitisers
Low molecular weight (200-500)
Planar, Tricyclic or Polycyclic Often contain heteroatoms
Stabilise resonance structures All absorb UV/visible radiation
Photosensitised Reaction Mechanism
Do 1D
3D
1O2 + Do
Do + A• H•
D- + A• H+
orA-OO-H
AH3O2
absorption
fluorescence orInternal conversion
singlet excited state
triplet excited state
singlet oxygen
intersystem crossings
phosphorescenceor intersystem crossings
ground stateabsorbs light
AH
e-promoted to next energy levele- spin state not changed
O2 in ground state, exists as a triplet
peroxy molecule
Type IIType I
Free radical
Photodynamic action
Type 1 Energy is sufficient to cause dissociation of
reaction molecule loses H
Free radical species - reactive often undergoes reaction with 3O2 to produce a
peroxidised molecule chain reaction
Type 2 Photosensitised oxidation
Photodynamic Action
Internal conversion Interaction between energy levels Molecules can dissipate energy via vibration
relaxation, returns to original state No chemical change to molecule
Fluorescence Molecule gives up photon of light as it returns
to ground statePhosphoresence
Molecule gives up photon of light as it returns to ground state from triplet excited state
Acceptors for Photodynamic
ActionProteins - amino acids eghistidine
Tryptophan - types I (free radical) and types II 1O2
Type II reaction
Imidazole ring reacts with 1O2 adding O2 across the double bond, forming an unstable cyclic intermediate which subsequently breaks down
Histidine in protein gets damaged
+
Acceptors for Photodynamic
ActionLipids
Type II reaction1O2adds across double bond of lipid molecule Lipids are found in cell membrane, reaction leads to disruption of the cell membrane and cell death
Acceptors for Photodynamic
ActionCarbohydrates - alcohols, sugars, vitamin C
Type I reactionContain many hydroxyls (-OH) which interact with the triplet excited state giving a free radical reactionA free radical is formed and can react further
alcohol ketone
Acceptors for Photodynamic
ActionNucleic Acids - Purines
Guanine, xanthine + 1O2
Purines can be oxidised and therefore are susceptible to attack by singlet oxygen.Not as susceptible as histidine or tryptophan, but damage does occur
Type II damage
Photosensitisers Absorbs Sunlight Energy Pathological response
in Skin
Endogenous Photosensitisers» Porphyrins complex with Fe in Haemoglobin» Disease state - Porphyria (excess unbound porphyrins
in circulation (400-450 nm)
Exogenous Photosensitisers» Contact
Tars and psoralens, perfumes, plants» Systemic
Psoralens and drugs (320-400 nm)
Photochemical Activity and
Phototoxicity
Drug or pollutant
1O2prod. (1)
Free radical generation (2)
Phototoxicity in Mouse (3)
Clinical Reports of Phototoxicity (4)
8-Methoxypsoralen
37 39 strong Very many
Cloropromazine 54 36 strong many
Pomazine 20 17 moderate few
Hydrochlorothiazide
13 14 strong many
Frusemide 40 22 strong many
Nalidixic acid 200 18 strong many
Cloroquine 18 14 weak few
Diazepam 4 <0.1 weak few
Chlorodiazepoxide
2 <0.1 weak few
Qinine 96 2 weak few
Metronidazole 0 scavenger
moderate few
Azathioprine 1 scavenger
moderate many
Benoxaprofen 37 42 strong Very many
Naproxen 15 11 moderate some
Indometacin <1 <0.1 none few
Oxytetracycline 11 0.5 moderate some
Demeclocycline 13 5 strong many
Dimethylbenzantracene
232 3 Strong (topical)
many
Benzacridine 185 2 strong manyComparison of fundamental photochemical activity with mouse
phototoxicity tests and clinical photosensitivity responses(1) Based on O2 uptake and flash photolysis measurements. (2) Based on polymerisation and electron spin resonance experiments. (3) Adapted from various literature reports (4) Adapted from ADRAC and Magnus
Photsensitisation
UV + Photosensitising Chemical
Excited State Chemical
Loss of Energy by Fluorescence Phosphorescence Internal Conversion
Photochemical Reactions
Photochemical Reactions
1. Energy transfer to molecular oxygenExcited singlet oxygen oxidation of lipids or proteins
2. Energy transfer to BiomoleculeOxidation of excited state biomolecules
3. Covalent binding to BiomoleculeAltered DNA, Lipid or Protein
4. Formation of Toxic Photoproduct
5. Binding to skin proteinFormation of Hapten as photoproductImmunological Sensitisation
Photsensitising Drug
Excited State Drug
Triplet State
Energy transfer to molecular O2
Excited singlet O2
Oxidation-peroxidation of biomolecule (lipid, protein)
Energy transfer to biomolecule
Oxidation of excited state biomolecule
Molecular change
Free Radicals
Formation of photoproduct(s)
Electron transfer or covalent binding to biomolecule
Molecular change to cell components
Photo-oxidation of cell components
Toxic reaction with cell components
Damage to critical cell components
Phototoxic Skin Response
Deactivation Mechanisms(Fluorescence, Internal conversion, etc)h
h
Naproxen
Peroxy radical
radical
Benoxaprofen
radical
Hydroxyl radical
Diclophenac
carbazole
photosubstitution
photoreduction
Secondary photoproduct
NSAIDS
Naproxen Ketoprofen Ibuprofen
SuprofenBenoxprofen
SulindacIndomethacin
Diflusinal
Diclofenac Piroxicam
Cellular Defence Mechanisms
Vitamin C and E Natural antioxidants
Uric AcidGlutathione
Protects mitochondria from free radicalsSuperoxide Dismutase
Catalyses reaction of toxic superoxide radical O2
-. to O2
Catalase Reduces hydrogen peroxide (HOOH)
Various oxidase - reductase systems
Cellular Defences
MechanismsVitamin A, carotenes
Contain long chains of conjugated double bonds
Other dietary Supplements Flavonoids
Depletion of defence mechanisms may lead to damage to cell macromolecules eg DNA
Can drugs initiate skin
cancer?
Repeated phototoxic injury leads to skin cancer in experimental animals
PUVA treatment (psoralens + UVA radiation) for psoriasis can cause skin cancer after prolonged use
increase in BCC
Immunosuppressive therapy with azathioprine is associated with high cancer incidence
In general population, phototoxic reactions may add to the already high risk of skin cancer from sunburn
Summary
Action of UV on biochemicals, eg proteins, DNA, sugars
Interaction of drugs with UV radiation resulting in photosensitisation or photoallergy
Photodynamic action Type 1 Type 2
Cellular defence mechanisms