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The relationships of human platelet thromboxane A2-prostaglandin H2 (TxA2/PGH2) receptor occupation as assessed by equilibrium binding of the TxA2/PGH2 agonist [125I]BOP to the functional responses of I-BOP-induced platelet shape change and aggregation were determined before and after specific, irreversible inactivation of platelet TxA2/PGH2 receptors with the photolyzable TxA2/PGH2 antagonist I-PTA-PON3. I-BOP stimulated platelet shape change and aggregation with concentrations producing a half-maximal response of 173 +/- 39 pM (n = 4) and 1.8 +/- 0.4 nM (n = 6), respectively (means +/- SE). Covalent inactivation of TxA2/PGH2 receptors with I-PTA-PON3 caused rightward shifts of I-BOP shape change and aggregation dose-response curves and resulted in pharmacological dissociation constants (Kd) of 134 pM and 1.95 nM, respectively. Isotherms of [125I] BOP binding to intact platelets (n = 6) were shallow with Hill coefficients of -0.68 +/- 0.03 and were best described by a two-site model with 222 +/- 58 sites/platelet of high affinity (Kd = 270 +/- 60 pM) and 818 +/- 90 sites/platelet of lower affinity (Kd = 3.9 +/- 1.2 nM). The relationship of I-BOP occupancy of high-affinity receptors to shape-change responses was linear (r = 0.97, P less than 0.001), but the occupancy-response relationship for the lower affinity receptor and platelet aggregation was hyperbolic with half-maximal aggregation occurring after occupation of 25% of the receptors. After covalent inactivation of a variable proportion of the receptors with I-PTA-PON3, the occupancy-response relationship for platelet aggregation resulted in a similar hyperbola indicating an excess of low-affinity receptors coupled to aggregation (spare receptors).(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract. Twenty-eight new aromatic esters of camptothecins 2-29 were prepared in yields of 5 to 96% by straight acylation of camptothecin (1a) and 9-nitrocamptothecin (1b) with various aromatic acids as acylating agents. All of these esters were tested against 14 different human cancer cell lines. The antitumor activity of these compounds was related to the nature of the substituting groups of their side aromatic chains. In general, esters with strong electron-withdrawing groups on their side aromatic chains were active; esters with halogen-substituted side aromatic chains were slightly active; and esters without any substituting groups on their side aromatic chains were practically inactive. The IC 50 studies showed that the majority of these esters were not as potent as their parental compounds 1a and 1b; whereas, the potencies of esters 6 and 25 were exceptionally high, much higher than the commercial camptothecin analogues and comparable to (or slightly more potent than) their parental compounds.

Human leukemia U‐937/WT cells were exposed to stepwise increased concentrations of Vincristine so that Vincristine‐resistant cell sublines (termed U‐937/RV) were developed. Established U‐937/RV cell sublines have continuously propagated over a year, both in absence and presence of VCR, and have demonstrated similar features. In contrast to U‐937/WT cells, U‐937/RV cells have longer doubling time, and are more differentiated as determined by appearance of distinct morphological features and synthesis of mRNA that codes for the monocyte colonystimulating factor‐1 receptor (c‐fms). Both apoptosis‐suppressing Bcl‐2 and Bcl‐XL proteins were undectable in U‐937/WT cells, whereas Bcl‐2 was nearly detectable and Bcl‐XL readily detectable in U‐937/RV cells. The apoptosis‐promoting Bax protein was also absent in U‐937/WT cells and readily detected in U‐937/RV cells. Vincristine‐resistant cells with different levels of resistance synthesize similar levels of c‐fms mRNA and Bax protein. Finally, unlike U‐937/WT cells, U‐937/RV cells have no ability to induce tumors when xenografted in immunodeficient mice. The findings collectively suggest that development of resistance to Vincristine in U‐937/WT cells may correlate with cell differentiation and synthesis of proteins that regulate apoptosis.