ELUCIDATING THE ROLE OF RETROACTIVE SIGNALING AND KINASE INHIBITORS ON OFF-TARGET DRUG EFFECTS

ALEJANDRA C VENTURA; MICHELLE WYNN; JACQUES-A. SEPULCHRE; HECTOR J GARCIA; SOFIA D MERAJVER

Orlando

Conferencia; Era of Hope 2011; 2011

Congressionally Directed Medical Research Programs

Background and Objectives: The primary objective of targeted cancer therapies is to modulate cancer progression by perturbing specific molecules involved in aberrant proliferation and invasion. Targeted therapies often take the form of kinase inhibitors, which are designed to interfere with a specific kinase molecule in a deregulated oncogenic signaling cascade. While extremely promising as anticancer agents, such inhibitors may have undesirable off-target effects, whether by nonspecific interactions or by direct pathway cross-talk effects. We have recently shown in published experimental and theoretical work that covalently modified signaling cascades naturally exhibit bidirectional signal propagation. This phenomenon is termed retroactivity and challenges the widespread notion that information in cascades only flows from the cell surface to the nucleus. This novel finding indicates that when a signal is transmitted forward from the cell surface to the nucleus, information is also transmitted backward without the need for explicit feedback connections. Previous work has demonstrated that increasing the concentration of a phosphatase in the terminal cycle of a signaling cascade may result in a measurable decrease in the concentration of an upstream active kinase. Thus, a downstream perturbation in a signaling cascade can produce a reverse (or retroactive) response without the need for direct negative feedback connections. This led us to hypothesize that the use of an inhibitory drug in a signaling network may propagate an upstream off-target effect.Methods: To test the hypothesis that retroactivity contributes to off-target drug effects, we developed several computational models for a series of signaling networks. The objective of our approach was twofold: (1) to probe the effect of retroactivity on a kinase inhibitor in a signaling network and (2) to test whether retroactivity is likely to produce a measurable off-target effect under physiologically realistic conditions. Specifically, our models simulate the targeted inhibition of an activated kinase in a series of multicycle networks. Parameter values in the computational models were taken from well-studied signaling networks.Results: The results of our work indicate that at physiologically and therapeutically relevant concentrations, a targeted inhibitor may induce a measurable off-target effect via retroactivity. We also performed local sensitivity analyses to predict the kinetic parameters with most effect on the off-target response and, surprisingly, the drug disassociation constant is predicted to have very little effect while parameters, such as enzyme saturation and maximum velocity of some cycles, are predicted to be very important.Conclusions: A proper characterization of a pathway’s structure is important for identifying which protein in the pathway represents the optimal drug target as well as what concentration of the targeted therapy is likely to modulate the pathway in the manner desired. We believe our results support the position that such characterizations should consider the role of retroactivity as source of a potential off-target effect by kinase inhibitors.