Poster L30 - Effects of Lymphocytes Trafficking Inhibition on Antiretroviral Drug Distribution within Lymph Nodes

Introduction: Antiretroviral (ARV) therapy is essential for managing HIV infection, primarily targeting the virus within blood plasma. However, the effective penetration of these drugs into secondary lymphoid tissues like lymph nodes (LNs) is crucial, as LNs are key reservoirs where HIV can remain hidden and persist even during therapy. ARV drugs can reach the LNs either through direct diffusion of unbound drugs across the high endothelial venules (HEVs) or via cell-mediated processes in which drugs are carried by lymphocytes. Understanding these pathways is vital for improving drug efficacy at these critical sites. To explore the relative contributions of free and cell-mediated drug trafficking, our collaborators have performed experiments in which lymphocyte traffic into the LN is blocked by a functional antibody.

In this work, we introduce an advanced pharmacokinetic (PK) model to examine the impact of blocking lymphocyte entry into the LNs—a condition induced by using a functional antibody—on the dynamics of local drug concentration in the blood and LNs. By comparing scenarios of normal and inhibited lymphocyte traffic, our model seeks to delineate the contributions of these transport mechanisms to the overall drug distribution within blood and LN. This approach not only illuminates the pharmacokinetics of ARVs in a critical compartment but also aids in understanding how changes in lymphocyte trafficking could enhance or impede drug delivery to areas critically affected by HIV.

Materials and

Methods: This study presents a sophisticated pharmacokinetic model to investigate the dynamics of antiretroviral drugs—efavirenz (EFV), ritonavir (RTV), and tenofovir (TFV)—across both systemic circulation and lymphatic systems. Non-linear differential equations were employed to describe the transport of each drug between these compartments. The model features multiple compartments: Plasma (bound and unbound), peripheral blood mononuclear cells, lymph node (LN) interstitial fluid (bound and unbound), and LN intracellular regions. It also tracks changes in total interstitial fluid volume due to alterations in lymphocyte transport.

EFV and RTV are modeled in both bound and unbound states across compartments. TFV, characterized by a minimal bound fraction, is modeled only in its free state. Model parameters for these drugs were extracted from pharmacokinetic studies involving HIV-infected patients. In some cases, due to data unavailability, assumptions were made based on existing literature and tested through sensitivity analyses.

The model simulated the reduced lymphocyte traffic into LNs induced by a functional antibody. To mimic the effect of functional antibody blocking of lymphocyte entry into the lymph nodes, our model uses a logistic growth model for the extracellular fraction of total volume in the LNs. The main outcomes examined were changes in drug concentrations over time within each compartment, focusing particularly on total and intracellular concentrations within LNs.

The model's predictions were validated using experimental data from collaborating laboratory. This approach provides a robust framework to understand the distribution mechanisms of ARV drugs within lymph nodes, key sites for HIV persistence and replication.

Results, Conclusions, and Discussions: The pharmacokinetic model for EFV, RTV, and TFV reveals that blocking lymphocyte entry into the LN, results in a significant increase in the extracellular volume over time. This change subsequently leads to a marked increase in intracellular drug concentration within the LN, compared to scenarios where lymphocyte entry is unimpeded. For EFV, while there is an increase in the intracellular drug concentration within the LN, the concentration in the extracellular compartment decreases. However, the total drug concentration within the LN remains unchanged, likely a consequence of the reversible transport of EFV between extracellular and intracellular compartments. In the case of TFV, the decrease in extracellular concentration outweighs the increase in intracellular, resulting in a net reduction in total LN TFV concentration. For RTV, unlike EFV and TFV, the extracellular drug concentration remains unchanged, and the total concentration of RTV within the LN increases. Both TFV and RTV exhibit irreversible transport of drug from extracellular to intracellular compartments; this allows for the creation of a net imbalance in the total drug delivered to the LN due to changes in lymphocyte trafficking.

Our model predicts that blocking lymphocyte entry into the LN compartment leads to increased intracellular drug concentrations within the LN, a result confirmed experimentally by our collaborators in the case of EFV and TFV. This led us to investigate whether enhancing lymphocyte entry into the LN would conversely reduce intracellular drug concentrations. To test this hypothesis, we employed a non-linear PK model that incorporates the decrease in interstitial fluid volume fraction due to increased lymphocyte entry. Our findings indicated that increased lymphocyte entry significantly reduced the intracellular drug concentration. Furthermore, we observed that the extracellular and total LN drug concentration effects were inverse to those observed with blocked lymphocyte entry case. These results suggest that swollen lymph nodes may impair the distribution of drug, potentially compromising the efficacy of HIV treatment. Moreover, our analysis reveals that obstructing or enhancing the entry of lymphocytes into the LN has no significant impact on blood concentrations. These non-intuitive findings emphasize the importance of considering extracellular volume dynamics in ARV drug transport studies.

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