Paclitaxel is extensively metabolized in the liver. Metabolism of paclitaxel to its major metabolite, 6alpha-hydroxypaclitaxel, is mediated by cytochrome P-450 isoenzyme CYP2C8,1 185 187 202 354 while metabolism to 2 of its minor metabolites, 3'-p-hydroxypaclitaxel and 6alpha,3'-p-dihydroxypaclitaxel, is catalyzed by CYP3A4.
The elimination of nonradioactive taxol in bile and urine was investigated in the rat after administration via the caudal vein (10 mg/kg). As in humans, no metabolites of taxol were detected by HPLC in rat urine, and only 10% of the injected taxol was recovered in urine over a 24 hr period. In contrast, 11.5% and 29% of the injected taxol was recovered in rat bile as unchanged taxol and metabolites, respectively. Among the nine taxol metabolites detected by HPLC, the side chain at C13, which is required for pharmacological activity, had been removed in only one minor metabolite, baccatin III. The chemical structures of the two major hydroxylated metabolites were determined by MS (fast atom bombardment and desorption chemical ionization) and (1)H NMR spectroscopy. One was a taxol derivative hydroxylated on the phenyl group at C3 of the side chain at C13, while the other corresponded to a taxol derivative hydroxylated in the m-position on the benzoate of the side chain at C2. Although these two major taxol metabolites were as active as taxol in preventing cold microtubule disassembly, they were, respectively, 9 and 39 times less cytotoxic as taxol on in vitro L1210 leukemia growth. These results show for the first time that there is a significant hepatic metabolism of taxol.
来源:Hazardous Substances Data Bank (HSDB)
代谢
为了研究紫杉烷类化合物C3'位取代基对其代谢的影响，比较了一对在C3'位略有不同的类似物cephalomannine和paclitaxel的代谢。在用人类肝脏微粒体和NADPH生成系统孵化cephalomannine后，通过液相色谱/串联质谱检测到了两种单羟基代谢物（M1和M2）。C4''（M1）和C6alpha（M2）被提出作为可能的羟基化位点，M1的结构通过(1)H NMR得到确认。化学抑制研究和重组人细胞色素P450（P450s）的酶活性实验表明，4''-hydroxycephalomannine主要由CYP3A4生成，而6alpha-hydroxycephalomannine主要由CYP2C8生成。Paclitaxel和cephalomannine的整体生物转化率略有差异（184 vs. 145 pmol/min/mg），但在五个人类肝脏样本中，C13侧链与C6alpha羟基化代谢物的平均比率差异显著（15:85 vs. 64:36）。与paclitaxel相比，cephalomannine的主要羟基化位点从C6alpha转移到了C4''，主要代谢酶从CYP2C8变为了CYP3A4。在大鼠或小型猪肝脏微粒体的孵化系统中，仅检测到了4''-hydroxycephalomannine，其形成被CYP3A抑制剂所抑制。AutoDock的分子对接表明cephalomannine采取了有利于4''-羟基化的取向，而paclitaxel采取了有利于3'-p-羟基化的取向。动力学研究表明，由于V(m)的增加，CYP3A4催化cephalomannine的效率高于paclitaxel。我们的结果表明，紫杉烷在C3'位的相对较小的修饰对其代谢产生了重大影响。
To investigate how taxane's substituents at C3' affect its metabolism, ... the metabolism of cephalomannine and paclitaxel, a pair of analogs that differ slightly at the C3' position /was compared/. After cephalomannine was incubated with human liver microsomes in an NADPH-generating system, two monohydroxylated metabolites (M1 and M2) were detected by liquid chromatography/tandem mass spectrometry. C4'' (M1) and C6alpha (M2) were proposed as the possible hydroxylation sites, and the structure of M1 was confirmed by (1)H NMR. Chemical inhibition studies and assays with recombinant human cytochromes P450 (P450s) indicated that 4''-hydroxycephalomannine was generated predominantly by CYP3A4 and 6alpha-hydroxycephalomannine by CYP2C8. The overall biotransformation rate between paclitaxel and cephalomannine differed slightly (184 vs. 145 pmol/min/mg), but the average ratio of metabolites hydroxylated at the C13 side chain to C6alpha for paclitaxel and cephalomannine varied significantly (15:85 vs. 64:36) in five human liver samples. Compared with paclitaxel, the major hydroxylation site transferred from C6alpha to C4'', and the main metabolizing P450 changed from CYP2C8 to CYP3A4 for cephalomannine. In the incubation system with rat or minipig liver microsomes, only 4''-hydroxycephalomannine was detected, and its formation was inhibited by CYP3A inhibitors. Molecular docking by AutoDock suggested that cephalomannine adopted an orientation in favor of 4''-hydroxylation, whereas paclitaxel adopted an orientation favoring 3'-p-hydroxylation. Kinetic studies showed that CYP3A4 catalyzed cephalomannine more efficiently than paclitaxel due to an increased V(m). Our results demonstrate that relatively minor modification of taxane at C3' has major consequence on the metabolism.
Paclitaxel has been associated with serum aminotransferase elevations in 7% to 26% of patients, but values greater than 5 times the upper limit of normal (ULN) in only 2% of those receiving the highest doses. Similar rates of alkaline phosphatase elevations and occasional mild bilirubin elevations also occur. The abnormalities are usually asymptomatic, mild and self-limited, rarely requiring dose modification or discontinuation. Paclitaxel has not been linked convincingly to instances of delayed, idiosyncratic clinically apparent liver injury with jaundice. However, the hypersensitivity reactions that occur with infusions of paclitaxel can be severe and accompanied by acute hepatic necrosis. The liver injury may be relatively mild and anicteric (Case 1), but can also be severe with rapid onset of multiorgan failure and death. At least one instance of acute liver failure following a hypersensitivity reaction to paclitaxel has been published in the literature and recent modifications of the product labels for paclitaxel and docetaxel mention the occurrence of toxic deaths following severe infusion reactions. Because paclitaxel is often given with other antineoplastic agents, liver injury arising during therapy cannot always be reliably attributed to paclitaxel rather than to other specific agents. Furthermore, paclitaxel in combination with other anticancer agents may be associated with reactivation of hepatitis B, increased risk of opportunistic viral infections, sinusoidal obstruction syndrome or sepsis, any of which can cause liver test abnormalities or clinically apparent liver injury.
Concomitant administration of CNS depressants such as antihistamines or opiates with paclitaxel should be undertaken with caution as these drugs may cause potentiation of CNS depression caused by the alcohol contained in the paclitaxel formulation.
Paclitaxel bound to nanoparticles of the serum protein albumin is delivered via endothelial transport mediated by albumin receptors, and the resulting concentration of paclitaxel in tumor cells is increased compared with that achieved using an equivalent dose of conventional paclitaxel. Like conventional paclitaxel, albumin-bound paclitaxel has a large volume of distribution. Following 30-minute or 3-hour IV infusion of 80-375 mg/sq m albumin-bound paclitaxel, the volume of distribution averaged 632 L/sq m. The volume of distribution of albumin-bound paclitaxel 260 mg/sq m by 30-minute IV infusion was 53% larger than the volume of distribution of conventional paclitaxel 175 mg/sq m by 3-hour IV infusion. /Paclitaxel (albumin-bound)/
Following IV administration, paclitaxel is widely distributed into body fluids and tissues. Paclitaxel has a large volume of distribution that appears to be affected by dose and duration of infusion. Following administration of paclitaxel doses of 135 or 175 mg/sq m by IV infusion over 24 hours in patients with advanced ovarian cancer, the mean apparent volume of distribution at steady state ranged from 227-688 L/sq m. The steady-state volume of distribution ranged from 18.9-260 L/sq m in children with solid tumors or refractory leukemia receiving paclitaxel 200-500 mg/sq m by 24-hour IV infusion. Paclitaxel does not appear to readily penetrate the CNS, but paclitaxel has been detected in ascitic fluid following IV infusion of the drug. It is not known whether paclitaxel is distributed into human milk, but in lactating rats given radiolabeled paclitaxel, concentrations of radioactivity in milk were higher than those in plasma and declined in parallel with plasma concentrations of the drug.
For the dose range 80-375 mg/sq m, increase in dose of albumin-bound paclitaxel was associated with a proportional increase in AUC.354 The duration of infusion did not affect the pharmacokinetic disposition of albumin-bound paclitaxel. Following 30-minute or 3-hour IV infusion of albumin-bound paclitaxel 260 mg/sq m, the peak plasma concentration averaged 18,741 ng/mL. /Paclitaxel (albumin-bound)/
Peak plasma concentrations and areas under the plasma concentration-time curve (AUCs) following IV administration of paclitaxel exhibit marked interindividual variation. Plasma concentrations of paclitaxel increase during continuous IV administration of the drug and decline immediately following completion of the infusion. Following 24-hour IV infusion of paclitaxel at doses of 135 or 175 mg/sq m in patients with advanced ovarian cancer, peak plasma concentrations averaged 195 or 365 ng/mL, respectively; the increase in dose (30%) was associated with a disproportionately greater increase in peak plasma concentration (87%), but the increase in AUC was proportional. When paclitaxel was administered by continuous IV infusion over 3 hours at doses of 135 or 175 mg/sq m in patients with advanced ovarian cancer, peak plasma concentrations averaged 2.17 or 3.65 ug/mL, respectively; the increase in dose (30%) was associated with disproportionately greater increases in peak plasma concentration (68%) and AUC (89%).
A new 2-steps synthesis of 2′-epi-Taxol starting from natural Taxol• is described. The approach is centred on the formation of oxazoline on the side chain with concomitant inversion of the C-2′ followed by acid hydrolysis.
