Half Lives of The Rich and Poisonous

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 Molecular Structure

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.

Herceptin Molecule

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)
1 1 1 0.00 4 4.00
2 2 8 1.67 2 3.67
3 3 15 1.53 2 3.53
4 4 22 1.47 2 3.47
5 5 29 1.45 2 3.45
6 6 36 1.44 2 3.44
7 7 43 1.43 2 3.43
8 8 50 1.43 2 3.43
9 9 57 1.43 2 3.43
10 10 64 1.43 2 3.43
11 11 71 1.43 2 3.43
12 12 78 1.43 2 3.43
13 13 85 1.43 6 7.43
14 16 106 3.13 6 9.13
15 19 127 3.85 6 9.85
16 22 148 4.16 6 10.16
17 25 169 4.28 6 10.28
18 28 190 4.34 6 10.34
19 31 211 4.36 6 10.36
20 34 232 4.37 6 10.37
21 37 253 4.38 6 10.38
22 40 274 4.38 6 10.38
23 43 295 4.38 6 10.38
24 46 316 4.38 6 10.38
25 49 337 4.38 6 10.38
26 52 358 4.38 6 10.38

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