Re: Enbrel (etanercept) - how long does it stay inthe body?

[Guest guest]

You're welcome, a. As you can see, a lot depends on what the actual

half-life of Enbrel is for you. If it is 70 hours, there isn't very much

remaining after 11 or 12 days.

For people who notice that they could use injections closer together,

it's probably a safe bet that their half-life is on the shorter end of

the range.

From this discussion you can also see why Remicade (infliximab) is more

of a problem than Enbrel if something goes wrong. I've seen the

half-life for Remicade listed as anywhere from 8.5 to 12 days.

Re: [ ] Enbrel (etanercept) - how long does it stay

inthe body?

> Thanks for the great info . I'm glad Terri brought this up. I

didn't

> realize that Enbrel stays in the body so long. You've given me a much

> better understanding of the half lives of drugs.

> a

>

>

> > a and Terri,

> >

> > I took a stab at this question, and, try as I might, I couldn't

keep my

> > answer short, LOL. I don't know what reference Terri's PCP used to

come

> > to this conclusion, but I'm pretty sure it's based on Enbrel's

plasma

> > half-life (there are other types of half-life measures, but I will

limit

> > this discussion to plasma half-life). Maybe you could ask your PCP

how

> > he/she came to the conclusion that Enbrel remains in the body for

10-12

> > weeks, Terri?

> >

> > From everything I've read and VERY generally speaking, biologics

like

> > Enbrel (etanercept) are thought to be quite biodegradable and

cleared

> > by the body fairly easily. We don't have to worry about them

lingering

> > in the tissues and recirculating for extremely long periods of time

> > like, for example, the non-biologic Arava (leflunomide) does. Even

> > though Arava's half-life is around 2 weeks, it may persist in the

body

> > for up to 2 years! That's an advantage if it's working well and not

> > causing other problems, but a very serious issue if toxicity

occurs -

> > hence, the washout procedure available that is recommended for those

who

> > must discontinue Arava.

> >

> > How long a drug remains in the body depends on the drug's

properties,

> > half-life being one of the most important of them, and the person

> > receiving the drug. Everyone's own particular circumstances - state

of

> > health, other medications, individual make-up, and so forth - will

> > influence how long the drug will remain in the body. For this

> > discussion, I'll assume that Enbrel's half-life is the most

important

> > factor.

> >

> > I'm also going to make one more assumption: we're talking about

someone

> > who has reached a steady-state plasma concentration of Enbrel.

> >

> > By definition, the plasma half-life is the amount of time it takes

for

> > only half of the original amount of a drug to remain in the plasma.

That

> > is, after one half-life, 50% of the drug is eliminated from the

plasma,

> > 50% is left.

> >

> > After the next half-life (the 2nd one), half of the remaining 50%

(25%

> > of the original amount) is lost (75% of what was there originally is

now

> > gone).

> >

> > After 3 half-lives, 12.5% more is lost (87.5% of the drug is

gone).

> > After 4 half-lives, 6.25% more is lost (93.75% of the drug is

gone).

> > After 5 half-lives, 3.125% more is lost (96.875% of the drug is

gone).

> > After 6 half-lives, 1.563% more is lost (98.438% of the drug is

gone).

> > After 7 half-lives, 0.781% more is lost (99.219% of the drug is

gone).

> >

> > So, as a practical example, let's say you've been on Enbrel for

awhile

> > and you give yourself a 25 mg injection on the 1st of January.

> >

> > We'll assume that the entire 25 mg is available in the plasma to

begin

> > with (and we'll disregard whatever is left from previous injections

so

> > we can keep things simple).

> >

> > After the 1st half-life, 12.5 mg is left.

> > After the 2nd half-life, 6.25 mg is left.

> > After the 3rd half-life, 3.125 mg is left.

> > After the 4th half-life, 1.563 mg is left.

> > After the 5th half-life, 0.781 mg is left.

> > After the 6th half-life, 0.391 mg is left.

> > After the 7th half-life, 0.195 mg is left.

> >

> > But how long is each half-life?

> >

> > If you read the manufacturer's findings from the clinical trials

(and

> > the link a gave has this information:

> > http://www.rxlist.com/cgi/generic/etanercept_cp.htm), the half-life

is

> > given as: " median half-life of 115 hours (range 98 to 300 hours " ).

> > That's an average of about 4.8 days. So, on average, you would be

> > expected to lose 50% of the amount of Enbrel available at the

beginning

> > of the half-life cycle every 4.8 days.

> >

> > 1 half-life = 4.8 days

> > 2 half-lives = 9.6 days

> > 3 half-lives = 14.4 days

> > 4 half-lives = 19.2 days

> > 5 half-lives = 24.0 days

> > 6 half-lives = 28.8 days

> > 7 half-lives = 33.6 days

> >

> > So, in around 33.6 days (7 half-lives), on about February 3rd, you

would

> > have about 0.195 mg of the 25 mg of Enbrel you injected on January

1st

> > left in your plasma. Even at the 4th half-life you only have 1.56 mg

> > left and that would be only January 20th. Keep in mind that we are

only

> > talking about that one injection and, in reality, the prior

injections

> > would contribute to the total amount of Enbrel in the body also.

> >

> > What if you are one of those people listed in that range of

half-lives

> > from the study data provided by the manufacturer and your half-life

is

> > 300 hours (longer and more problematic if you wish to discontinue

Enbrel

> > for an infection)? That means each half-life cycle is 12.5 days. The

4th

> > half-life is at 50 days, the 5th half-life is at 62.5 days, the 6th

> > half-life is at 75 days (10.7 weeks), and the 7th half-life occurs

after

> > 87.5 days or 12.5 weeks. Maybe the 6th and 7th half-life cycles in

this

> > example is where Terri's doctor drew the conclusion that Enbrel

remains

> > in one's system for 10 - 12 weeks (which is the worst-case scenario

> > according to the study data).

> >

> > Other sources list Enbrel's half-life as an average of 70 hours, so,

if

> > that's true, we would have a half-life cycle of 2.9 days. The 4th

> > half-life would occur at 11.6 days, the 5th at 14.5 days, the 6th at

> > 17.4 days, and the 7th at 20.3 days. So, for a half-life of 70

hours,

> > Enbrel would leave your system very quickly after you stopped taking

it.

> >

> > Even if you have some of the Enbrel left in your body, just how much

is

> > enough to cause trouble should you have an infection? I don't know,

but

> > I'm guessing that by the time you are at the 4th half life of the

last

> > injection you took and beyond, the small amount left in your body

> > shouldn't be a problem. Enbrel doesn't stop you from producing TNF

> > (which is a cytokine key to fighting infection), rather it binds to

TNF

> > available in circulation and reduces TNF activity. The less Enbrel

you

> > have left, the less it is able to interfere with TNF. And,

> > theoretically, those with RA are making more TNF than the average

> > person.

> >

> >

> >