For the past few weeks, I have been wondering how long the Taxol and Herceptin stay in my system after Wednesday infusion appointments. Here’s what I calculated after some reading online.
Taxol has an average elimination half-life of 2.33 hours for 3 hour infusions with a dosage of 175mg/m2. My Taxol infusion only takes 1 hour with a dosage of 80mg/m2 but since I couldn’t find a half-life specific to my dosage, I’ll approximate with the above figure.
We assume half of the Taxol’s pharmacological activity is completed by 2.33 hours.
Hence, 3/4 is gone by 2 x 2.33 = 4.66 hours. 7/8 by 6.99 hours. 15/16 by 9.32 hours. 31/32 by 11.65 hours. Before I awaken on Thursday morning, the Taxol has become almost inert or completely eliminated. The Taxol ultimately leaves the body primarily via solid waste.
I wear the cold caps for 4 hours past the end of the Taxol infusion. At that point,
0.5*(1-0.5*((4-2.33)/2.33)) = 0.32 of the original dose is intact.
In other words, when I stop using the Cold Caps, 32% of the Taxol dose is still at work inside me.
After the first 4mg/kg loading dose of Herceptin in Week 1, the dosage for Weeks 2-12 is 2 mg/kg. On this schedule, Herceptin has an average half-life of 6 days.
On Day 8 at the time of my 2nd infusion, there was roughly 0.5*(1-0.5*((8-6)/6)) = 0.42 of the 4mg/kg dose left, or 1.67 mg/kg.
Adding the 2nd infusion’s 2mg/kg dose to the residual 1st dose left me with 3.67 mg/kg.
Infusions 3-12 can be calculated similarly, with results shown in the table below.
The dropoff in Herceptin levels plateaus starting from Week 7 and remains constant through Week 12 at 3.43 mg/kg.
After Week 12, I discontinue the Taxol and continue the Herceptin alone for another 9 months. The infusions switch to a 3 week schedule at a 6mg/kg dosage, which exhibits an average 16 day half-life. As you can see in the table below, the Total Dose jumps to 7.43 mg/kg in Week 13 and eventually reaches a steady state of 10.38 mg/kg on Day 253 (Week 36). The 3x higher dosage would assumably correlate with more severe side effects, which I won’t be able to verify or refute until spring 2012.
Herceptin Schedule and Dose Level
|Infusion #||Week||Day||Leftover Dose (mg/kg)||New Dose (mg/kg)||Total Dose (mg/kg)|
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