Q and A

Question

Are drug levels in penile tissue important for PrEP?

I read about PrEP levels having to be at good levels in vaginal and rectal tissue but can’t find any references to drug levels in the penis.

If a man is only the active partner will event-based dosing be okay?

Answer

Answer: Simon Collins

Hi

Thanks for your question.

Event based dosing is fine for men whether or not they are top or bottom because it will achieve good drug levels thoughout the body. For men the important sites are in lymph tissue in the anus and groin.

For a man whose risk is only insertive – whether with a man or woman – any risk is likely to relate to how long infectious fluids are in contact with penile tissue. This includes the foreskin and glans – which are not skin but mucous membrane that are easier for HIV to get through. (This pic shows different tissue types and HIV risk)

PrEP doesn’t need to be at high levels in penile tissue because it is in the local lymph nodes – ie in the groin – where HIV develops. So even though the infection happens through the foreskin or glans, it takes hold in the lymph nodes and this is the tissue which need good drug levels.

The way drugs are process by the body is quite complicated though and varies for different drugs (and different people).

Tenofovir is interesting for reaching very high level in rectal tissue – about 10 x higher than in vaginal tissue and 100 times higher than in blood. This is why event-based dosing is good enough for receptive anal sex but not receptive vaginal sex. These internal tissues are also much closer to and include lymph nodes which is where HIV infection get hold and multiplies.

The evidence to support this answer comes from the lack of HIV transmission in PrEP studies where men have clearly been both active and partners.

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