Pegintron & Pegasys. Dosage inconsistencies.

[GoodByeHepC]

I've been doing my best to inform myself about HCV meds and there are several questions that I just can't find answers to - including from my Dr.

- Why is the Pegasys dosage so much higher than Pegintron and have only one dosage option?

- Why is the mono-therapy dosage for Pegintron LOWER than the combination treatment dosage?

If anyone has stumble upon the answers to these questions I'd really love to know.

thanks

[zazza]

I was hoping somebody better at it than I would answer this question, but since nobody does I will give it a shot. What I have heard is that Pegasys has just one dosage because it has larger molecules than PegIntron and therefore mainly stays in your bloodstream while PegIntron with its smaller molecules can find its way where Pegasys can not. Every adult has the same amount of blood independant of body weight, and therefore everyone should have the same amount of Pegasys.

One is also not to compare milligrams of Pegasys with milligrams of PegIntron. They are not the same.

At least my answer will bump this thread up, and perhaps somebody else who is better informed can add to it. It is an interesting question.

Concerning the mono-therapy dosage for PegIntron I would think they upped the dosage nowadays to get more people to SVR. Just my guess.

[zazza]

5 liter is the amount of blood in an adult.

[FoieGras]

OK, I'll take a try....

Pegasys is Interferon alfa-2a with a chunk of an inert molecule attatched. The molecular weight of the chunk is about 40,000 Daltons. The inert material is polyethylene glycol(PEG), a long-chain molecule or polymer which passes unmodified through the body.

Pegetron/PegIntron is Interferon-2b with a different-sized chunk of PEG on it., I believe its 14,000 Daltons.

(You will recall that 1 Dalton is the weight of a hydrogen atom. A water molecule weighs 18 Daltons)


The PEG acts to delay aborption and metabolism of the drug, so it can be given weekly instead of daily, which is how in the old days, interferon was given - daily injection. (Some interferons are still given daily)

First, the dosages in milligrams/micrograms of the two drugs cannot be directly compared: they are different molecules. Apples and oranges.

Second, the Pegasys is BIG and slowly absorbed from the injection site and carried to the liver where it acts on liver cells. Its level in the blood rises and stays high for 4-7 days.

On the other hand, PegIntron is a smaller more fragile molecule and is fairly rapidly metabolized and excreted. Its level in the blood rises during the first 2-3 days after injection and quickly falls so that by day 5 - day 7 you may have NO drug in yr body.

You can minimize the effect of this by increasing the dose or increasing how often you give the drug. Schering has chosen to use weight-based dosing; a higher weekly dose bumps up the average drug level and may prevent levels falling to zero.

Plus, the studies show weight-based dosing works!

Schering, in my opinion, has made a calculation that by  using higher doses for bigger people they can avoid giving the drug more than once weekly and so compete with Roche, makers of Pegasys. Personally, I think the dose should be given more frequently, the course I am following in my treatment.

Can't help ya with the monotherapy question.

Good Luck!

[GoodByeHepC]

Thanks for the info Zazza & FoieGras,

The different sized Peg molecule is very interesting. That's a bit of new info for me. Thanks for that. Would you happen to remember where you saw that data?

But, wouldn't it be logical to assume that if the Pegasys molecule was larger and more efficiently spreads out the absorption of the interferon then the dosage would be SMALLER  than pegintron... right?

If I understand your explanation the Pegintron is depleted more quickly and therefore it should be the Pegintron which is delivered in higher doses.

As far as not being able to compare the interferon portion of each drug goes... That's exactly the data I'm looking for. Is there more than one type of actual interferon? If so why is there no discussion anywhere about it.

And of course the question of mono therapy Vs Combo therapy with Pegintron?

I can't imagine why the dosage for Mono therapy for Pegintron is LOWER than the combo therapy.

Is it just a statistical mishap that somehow made it past the FDA? Or is there hard science out there that can explain it?

I'm actually very surprised that more of us being treated are not pressing this issue.

Thanks!

[rochammer]

mono-therapy ½ dose is now used for maintenance. I the point is to keep the vl low to stop damage to the liver not to get to svr.

The volume of the dose is mostly water. It’s the % of medication that is different. Pegintron  is weight based just like the riba. Pegasus is one size fit all.

Yes different sized Peg molecule is very interesting but which is better no one knows, the seem to work about the same, I did both relapsed on Pegasus 24 weeks tx.  Svr on pegintron 48 weeks tx. I think the most important thing is that they are both good meds and one may work better depending on the body chemistry of each person. Pegintron is smaller so maybe it can get into places that Pegasus can’t no one knows for sure.

The ½ life of Pegasus is longer than pegintron but they both stay in you system to some point for about week.

Bottom line is they both work, My doc could not really the same questions when I asked him a few years ago. The thing is he said they both work and none of that stuff really matter with the tx.

Are you going to treat soon?


Good luck rock  

[Marcia2202]

The way I understood I FoieGras' explanation, is that the actual strength of the medication is the same in Pegintron and Pegasys. They just don't have the same volume.

Marcia