Psoriasis may respond to PUVA therapy, in which the patient takes an oral psoralen (P) followed by ultraviolet A (UVA) exposure. The therapy should only be used in patients with severe psoriasis unresponsive to other therapy and should only be administered by physicians experienced in its use.
Oxsoralen-Ultra is a formulation of methoxsalen with greater bioavailability and faster onset of photosensitivity. A regimen using oxsoralen-ultra is not interchangeable with other psoralens or formulations of methoxsalen.
Parameters for initiating therapy:
(1) determining the initial dose of Oxsoralen-Ultra
(2) determining the UVA dose
(3) determining the exposure time in minutes
The first step in develop a regimen is to determine the minimum phototoxic dose (MPD), which is determined prior to any UVA exposure. The reference dose to start from is based on the person's body weight. This also is referred to as the minimal erythema dose (MED, Frutmann).
|
Body Weight in Kilograms |
Reference Dose of Oxsoralen-Ultra in mg |
|
< 30 |
10 |
|
30 - 50 |
20 |
|
51 - 65 |
30 |
|
66 - 80 |
40 |
|
81 - 90 |
50 |
|
91 - 115 |
60 |
|
> 115 |
70 |
The initial dose to use with UVA exposure is 50% of the minimum phototoxic dose.
|
Skin Type |
Features |
Recommended UVA Dose in Joules per square cm |
|
I |
always burns, never tans |
0.5 |
|
II |
usually burns, sometimes tans |
1.0 |
|
III |
sometimes burns, usually tans |
1.5 |
|
IV |
never burns, always tans |
2.0 |
|
V |
moderately pigmented |
2.5 |
|
V1 |
black |
3.0 |
where:
• A patient with erythrodermic psoriasis is started at an UVA dose as if Type I.
initial exposure time in minutes =
= (desired UVA dose in Joules per square cm) / (0.06 * (irradiance of the source in mW per square cm)
Subsequent dosing is determined by the response of the psoriasis and the level of adverse reactions. Dosing should not be given any more often than every other day, since this allows for evolution of any serious photoxicity.
Specialty: Dermatology, Immunology/Rheumatology
