Pharmacodynamics of Taxol in Human Head and Neck Tumors¹

The pharmacodynamics of taxol in human head and neck squamous cell carcinoma were studied using histocultures of surgical specimens from patients (n = 22). Tumors were treated with taxol for 24 h. The inhibition of DNA synthesis was determined by 48 h cumulative bromodeoxyuridine (BrdUrd) incorporation. The induction of apoptosis was measured by morphological changes, in situ DNA end labeling, post-exonuclease III-BrdUrd labeling, and DNA fragmentation. Inhibition of the BrdUrd labeling index (LI) by taxol was incomplete, with 11 tumors showing a maximal inhibition (E_max) of 30–50% and the remaining 11 tumors showing an E_max of 50–80%. For both groups, the inhibition approached maximum values at 1 µm taxol concentration; an additional 10-fold increase in drug concentrations did not significantly enhance the inhibition. The taxol concentrations required for a 30% inhibition (IC₃₀) were 4.2 and 0.3 µm for the first and second groups, respectively. The IC₃₀ correlated with the E_max (r² = 0.39; P < 0.001). Taxol induced apoptosis in all tumors; 11 tumors showed a maximal fraction of apoptotic tumor cells between 3 and 10% and 11 tumors between 13 and 28%, whereas untreated controls showed a maximal apoptotic index of <1%. For individual tumors, the maximal apoptotic index occurred between 0.1 and 3 µm, and correlated with the BrdUrd LI for the untreated control (r² = 0.37; P < 0.01). It is interesting that > 95% of apoptotic cells were BrdUrd labeled, whereas not all BrdUrd-labeled cells were apoptotic. To investigate the basis of the variable tumor response to taxol, we determined the expression of multidrug resistance P-glycoprotein (Pgp), p53, and bcl-2 proteins, using immunohistochemical staining and Western blot analysis. Eleven (50%), 10 (45%), and 7 (32%) tumors expressed Pgp, p53, and bcl-2, respectively. Patients with Pgp-positive tumors showed a higher number of affected lymph nodes than those with Pgp-negative tumors (P < 0.05). Compared with moderately and well differentiated tumors, the poorly differentiated tumors expressed p53 and Pgp more frequently and showed a lower maximum inhibition of DNA synthesis and a higher apoptotic fraction after taxol treatment (P < 0.05 in both cases). Pgp expression correlated differently with taxol-induced inhibition of DNA synthesis than with apoptosis; Pgp-positive tumors showed a significantly higher E_max (63%) and IC₃₀ (4.2 µm) but also a higher apoptotic index (17%) than Pgp-negative tumors (E_max, 36%; IC₃₀, 0.3 µm; and apoptotic index, 6%; P < 0.05 for all cases). p53 and bcl-2 expression did not correlate with taxol-induced inhibition of DNA synthesis or apoptosis. The data indicate that taxol acts through apoptosis and inhibition of proliferation in human head and neck cancer. Pgp overexpression appears to protect cells from the antiproliferative effect of taxol but correlates with a higher apoptosis.