E6201, an intravenous MEK1 inhibitor, achieves an exceptional response in BRAF V600E-mutated metastatic malignant melanoma with brain metastases

Molecular Targeting of the BRAF Proto-Oncogene/Mitogen-Activated Protein Kinase (MAPK) Pathway across Cancers

The mitogen-activated protein kinase (MAPK) pathway is essential for cellular proliferation, growth, and survival. Constitutive activation of this pathway by BRAF mutations can cause downstream activation of kinases, leading to uncontrolled cellular growth and carcinogenesis. Therefore, inhibition of BRAF and the downstream substrate MEK has been shown to be effective in controlling tumor growth and proliferation. Over the last decade, several BRAF and MEK inhibitors have been investigated, ranging from primarily melanoma to various cancer types with BRAF alterations. This subsequently led to several Food and Drug Administration (FDA) approvals for BRAF/MEK inhibitors for melanoma, non-small cell lung cancer, anaplastic thyroid cancer, colorectal cancer, histiocytosis neoplasms, and finally, tumor-agnostic indications. Here, this comprehensive review will cover the developments of BRAF and MEK inhibitors from melanomas to tumor-agnostic indications, novel drugs, challenges, future directions, and the importance of those drugs in personalized medicine.

Leptomeningeal Metastases in Melanoma Patients: An Update on and Future Perspectives for Diagnosis and Treatment

Leptomeningeal disease (LMD) is a devastating complication of cancer with a particularly poor prognosis. Among solid tumours, malignant melanoma (MM) has one of the highest rates of metastasis to the leptomeninges, with approximately 10-15% of patients with advanced disease developing LMD. Tumour cells that metastasise to the brain have unique properties that allow them to cross the blood-brain barrier, evade the immune system, and survive in the brain microenvironment. Metastatic colonisation is achieved through dynamic communication between metastatic cells and the tumour microenvironment, resulting in a tumour-permissive milieu. Despite advances in treatment options, the incidence of LMD appears to be increasing and current treatment modalities have a limited impact on survival. This review provides an overview of the biology of LMD, diagnosis and current treatment approaches for MM patients with LMD, and an overview of ongoing clinical trials. Despite the still limited efficacy of current therapies, there is hope that emerging treatments will improve the outcomes for patients with LMD.

Fungal Drug Discovery for Chronic Disease: History, New Discoveries and New Approaches

Fungal-derived drugs include some of the most important medicines ever discovered, and have proved pivotal in treating chronic diseases. Not only have they saved millions of lives, but they have in some cases changed perceptions of what is medically possible. However, now the low-hanging fruit have been discovered it has become much harder to make the kind of discoveries that have characterised past eras of fungal drug discovery. This may be about to change with new commercial players entering the market aiming to apply novel genomic tools to streamline the discovery process. This review examines the discovery history of approved fungal-derived drugs, and those currently in clinical trials for chronic diseases. For key molecules, we discuss their possible ecological functions in nature and how this relates to their use in human medicine. We show how the conservation of drug receptors between fungi and humans means that metabolites intended to inhibit competitor fungi often interact with human drug receptors, sometimes with unintended benefits. We also plot the distribution of drugs, antimicrobial compounds and psychoactive mushrooms onto a fungal tree and compare their distribution to those of all fungal metabolites. Finally, we examine the phenomenon of self-resistance and how this can be used to help predict metabolite mechanism of action and aid the drug discovery process.

Melanoma Brain Metastases: A Systematic Review of Opportunities for Earlier Detection, Diagnosis, and Treatment

Introduction: Melanoma continues to represent the most serious skin cancer worldwide. However, few attempts have been made to connect the body of research on advanced melanoma. In the present review, we report on strides made in the diagnosis and treatment of intracranial metastatic melanoma. Methods: Relevant Cochrane reviews and randomized-controlled trials published by November 2022 were systematically retrieved from the Cochrane Library, EMBASE, and PubMed databases (N = 27). Search and screening methods adhered to the 2020 revision of the Preferred Reporting Items for Systematic reviews and Meta-Analyses guidelines. Results: Although the research surrounding the earlier detection of melanoma brain metastasis is scarce, several studies have highlighted specific markers associated with MBM. Such factors include elevated BRAFV600 mutant ctDNA, high LDH concentration, and high IGF-1R. The approach to treating MBM is moving away from surgery and toward nonsurgical management, namely, a combination of stereotactic radiosurgery (SRS) and immunotherapeutic agents. There is an abundance of emerging research seeking to identify and improve both novel and established treatment options and diagnostic approaches for MBM, however, more research is still needed to maximize the clinical efficacy, especially for new immunotherapeutics. Conclusions: Early detection is optimal for the efficacy of treatment and MBM prognosis. Current treatment utilizes chemotherapies and targeted therapies. Emerging approaches emphasize biomarkers and joint treatments. Further exploration toward preliminary identification, the timing of therapies, and methods to ameliorate adverse treatment effects are needed to advance MBM patient care.

Systematic in vitro analysis of therapy resistance in glioblastoma cell lines by integration of clonogenic survival data with multi-level molecular data

Despite intensive basic scientific, translational, and clinical efforts in the last decades, glioblastoma remains a devastating disease with a highly dismal prognosis. Apart from the implementation of temozolomide into the clinical routine, novel treatment approaches have largely failed, emphasizing the need for systematic examination of glioblastoma therapy resistance in order to identify major drivers and thus, potential vulnerabilities for therapeutic intervention. Recently, we provided proof-of-concept for the systematic identification of combined modality radiochemotherapy treatment vulnerabilities via integration of clonogenic survival data upon radio(chemo)therapy with low-density transcriptomic profiling data in a panel of established human glioblastoma cell lines. Here, we expand this approach to multiple molecular levels, including genomic copy number, spectral karyotyping, DNA methylation, and transcriptome data. Correlation of transcriptome data with inherent therapy resistance on the single gene level yielded several candidates that were so far underappreciated in this context and for which clinically approved drugs are readily available, such as the androgen receptor (AR). Gene set enrichment analyses confirmed these results, and identified additional gene sets, including reactive oxygen species detoxification, mammalian target of rapamycin complex 1 (MTORC1) signaling, and ferroptosis/autophagy-related regulatory circuits to be associated with inherent therapy resistance in glioblastoma cells. To identify pharmacologically accessible genes within those gene sets, leading edge analyses were performed yielding candidates with functions in thioredoxin/peroxiredoxin metabolism, glutathione synthesis, chaperoning of proteins, prolyl hydroxylation, proteasome function, and DNA synthesis/repair. Our study thus confirms previously nominated targets for mechanism-based multi-modal glioblastoma therapy, provides proof-of-concept for this workflow of multi-level data integration, and identifies novel candidates for which pharmacological inhibitors are readily available and whose targeting in combination with radio(chemo)therapy deserves further examination. In addition, our study also reveals that the presented workflow requires mRNA expression data, rather than genomic copy number or DNA methylation data, since no stringent correlation between these data levels could be observed. Finally, the data sets generated in the present study, including functional and multi-level molecular data of commonly used glioblastoma cell lines, represent a valuable toolbox for other researchers in the field of glioblastoma therapy resistance.

The overview of Mitogen-activated extracellular signal-regulated kinase (MEK)-based dual inhibitor in the treatment of cancers

Mitogen-activated extracellular signal-regulated kinase 1 and 2 (MEK1/2) are the critical components of the mitogen-activated protein kinase/extracellular signal-regulated kinase 1 and 2 (MAPK/ERK1/2) signaling pathway which is one of the well-characterized kinase cascades regulating cell proliferation, differentiation, growth, metabolism, survival and mobility both in normal and cancer cells. The aberrant activation of MAPK/ERK1/2 pathway is a hallmark of numerous human cancers, therefore targeting the components of this pathway to inhibit its dysregulation is a promising strategy for cancer treatment. Enormous efforts have been done in the development of MEK1/2 inhibitors and encouraging advancements have been made, including four inhibitors approved for clinical use. However, due to the multifactorial property of cancer and rapidly arising drug resistance, the clinical efficacy of these MEK1/2 inhibitors as monotherapy are far from ideal. Several alternative strategies have been developed to improve the limited clinical efficacy, including the dual inhibitor which is a single drug molecule able to simultaneously inhibit two targets. In this review, we first introduced the activation and function of the MAPK/ERK1/2 components and discussed the advantages of MEK1/2-based dual inhibitors compared with the single inhibitors and combination therapy in the treatment of cancers. Then, we overviewed the MEK1/2-based dual inhibitors for the treatment of cancers and highlighted the theoretical basis of concurrent inhibition of MEK1/2 and other targets for development of these dual inhibitors. Besides, the status and results of these dual inhibitors in both preclinical and clinical studies were also the focus of this review.

An Insight into the Medicinal Chemistry Perspective of Macrocyclic Derivatives with Antitumor Activity: A Systematic Review

The profound pharmacological properties of macrocyclic compounds have led to their development as drugs. In conformationally pre-organized ring structures, the multiple functions and stereochemical complexity provided by the macrocycle result in high affinity and selectivity of protein targets while maintaining sufficient bioavailability to reach intracellular locations. Therefore, the construction of macrocycles is an ideal choice to solve the problem of “undruggable” targets. Inspection of 68 macrocyclic drugs on the market showed that 10 of them were used to treat cancer, but this structural class still has been poorly explored within drug discovery. This perspective considers the macrocyclic compounds used for anti-tumor with different targets, their advantages and disadvantages, and the various synthetic methods of them.

Dereplication of Fungal Metabolites by NMR-Based Compound Networking Using MADByTE

Strategies for natural product dereplication are continually evolving, essentially in lock step with advances in MS and NMR techniques. MADByTE is a new platform designed to identify common structural features between samples in complex extract libraries using two-dimensional NMR spectra. This study evaluated the performance of MADByTE for compound dereplication by examining two classes of fungal metabolites, the resorcylic acid lactones (RALs) and spirobisnaphthalenes. First, a pure compound database was created using the HSQC and TOCSY data from 19 RALs and 10 spirobisnaphthalenes. Second, this database was used to assess the accuracy of compound class clustering through the generation of a spin system feature network. Seven fungal extracts were dereplicated using this approach, leading to the correct prediction of members of both families from the extract set. Finally, NMR-guided isolation led to the discovery of three new palmarumycins (20-22). Together these results demonstrate that MADByTE is effective for the detection of specific compound classes in complex mixtures and that this detection is possible for both known and new natural products.

Emerging Studies of Melanoma Brain Metastasis

PURPOSE OF REVIEW

Among solid tumors, melanoma has the highest propensity for brain dissemination. Although newer treatment approaches have resulted in excellent control or elimination of brain metastasis in many patients, they remain the cause of significant morbidity and mortality. Here, we review recent preclinical and clinical studies to detail current understanding of the incidence, prognosis, biological characteristics, and treatments for melanoma brain metastases.

RECENT FINDINGS

Clinical trials tailored to this patient population have demonstrated prolonged disease control with immune checkpoint inhibitors. Emerging clinical challenges include radiation necrosis and perilesional edema, phenomena that are rarely seen in other organs. Recent preclinical studies have resulted in improved understanding of the tumor microenvironment in the brain, providing insights into additional treatment approaches. The biological basis of brain tumor homing and survival within the central nervous system remain understudied. Additional preclinical and clinical studies will enhance our ability to prevent and treat brain metastases.

HSP70 inhibition blocks adaptive resistance and synergizes with MEK inhibition for the treatment of NRAS-mutant melanoma

NRAS-mutant melanoma is currently a challenge to treat. This is due to an absence of inhibitors directed against mutant NRAS, along with adaptive and acquired resistance of this tumor type to inhibitors in the MAPK pathway. Inhibitors to MEK (mitogen-activated protein kinase kinase) have shown some promise for NRAS-mutant melanoma. In this work we explored the use of MEK inhibitors for NRAS-mutant melanoma. At the same time we investigated the impact of the brain microenvironment, specifically astrocytes, on the response of a melanoma brain metastatic cell line to MEK inhibition. These parallel avenues led to the surprising finding that astrocytes enhance the sensitivity of melanoma tumors to MEK inhibitors (MEKi). We show that MEKi cause an upregulation of the transcription factor ID3, which confers resistance. This upregulation of ID3 is blocked by conditioned media from astrocytes. We show that silencing ID3 enhances the sensitivity of melanoma to MEK inhibitors, thus mimicking the effect of the brain microenvironment. Moreover, we report that ID3 is a client protein of the chaperone HSP70, and that HSP70 inhibition causes ID3 to misfold and accumulate in a detergent-insoluble fraction in cells. We show that HSP70 inhibitors synergize with MEK inhibitors against NRAS-mutant melanoma, and that this combination significantly enhances the survival of mice in two different models of NRAS-mutant melanoma. These studies highlight ID3 as a mediator of adaptive resistance, and support the combined use of MEK and HSP70 inhibitors for the therapy of NRAS-mutant melanoma.

SIGNIFICANCE

MEK inhibitors are currently used for NRAS-mutant melanoma, but have shown modest efficacy as single agents. This research shows a synergistic effect of combining HSP70 inhibitors with MEK inhibitors for the treatment of NRAS mutant melanoma.

Synthetic Opportunities and Challenges for Macrocyclic Kinase Inhibitors

Macrocycles are typically cyclic variants of inhibitors derived from uncyclized canonical molecules or from natural products. For medicinal chemistry, drug-like macrocycles have received increasing interest over the past few years, since it has been demonstrated that macrocyclization can favorably alter the biological and physiochemical properties as well as selectivity in comparison to the acyclic analogue. Recent drug approvals such as Lorlatinib, glecaprevir, or voxilaprevir underline the clinical relevance of drug-like macrocycles. However, the synthesis of drug-like macrocycles can be challenging, since the ring-closing reaction is generally challenging with yields depending on the size and geometry of the bridging linker. Nevertheless, macrocycles are one opportunity to expand the synthetic toolbox for medicinal chemistry to provide bioactive molecules. Therefore, we reviewed the past literature of drug-like macrocycles highlighting reactions that have been successfully used for the macrocyclization. We classified the cyclization reactions by their type, ring-size, yield, and macrocyclization efficiency index.

The quiescent fraction of chronic myeloid leukemic stem cells depends on BMPR1B, Stat3 and BMP4-niche signals to persist in patients in remission

Chronic myeloid leukemia arises from the transformation of hematopoietic stem cells by the BCR-ABL oncogene. Though transformed cells are predominantly BCR-ABL-dependent and sensitive to tyrosine kinase inhibitor treatment, some BMPR1B⁺ leukemic stem cells are treatment-insensitive and rely, among others, on the bone morphogenetic protein (BMP) pathway for their survival via a BMP4 autocrine loop. Here, we further studied the involvement of BMP signaling in favoring residual leukemic stem cell persistence in the BM of patients having achieved remission under treatment. We demonstrate by single-cell RNASequencing analysis that a sub-fraction of surviving BMPR1B⁺ leukemic stem cells are co-enriched in BMP signaling, quiescence and stem cell signatures, without modulation of the canonical BMP target genes, but enrichment in actors of the Jak2/Stat3 signaling pathway. Indeed, based on a new model of persisting CD34⁺CD38^– leukemic stem cells, we show that BMPR1B⁺ cells display co-activated Smad1/5/8 and Stat3 pathways. Interestingly, we reveal that only the BMPR1B⁺ cells adhering to stromal cells display a quiescent status. Surprisingly, this quiescence is induced by treatment, while non-adherent BMPR1B⁺ cells treated with tyrosine kinase inhibitors continued to proliferate. The subsequent targeting of BMPR1B and Jak2 pathways decreased quiescent leukemic stem cells by promoting their cell cycle re-entry and differentiation. Moreover, while Jak2-inhibitors alone increased BMP4 production by mesenchymal cells, the addition of the newly described BMPR1B inhibitor (E6201) impaired BMP4-mediated production by stromal cells. Altogether, our data demonstrate that targeting both BMPR1B and Jak2/Stat3 efficiently impacts persisting and dormant leukemic stem cells hidden in their BM microenvironment.

MEK Inhibition Suppresses Growth of Atypical Teratoid/Rhabdoid Tumors

Atypical teratoid/rhabdoid (AT/RT) tumors are the most common malignant brain tumor of infancy and have a poor prognosis. We have previously identified very high expression of LIN28A and/or LIN28B in AT/RT tumors and showed that AT/RT have corresponding increased expression of the mitogen-activated protein (MAP) kinase pathway. Binimetinib is a novel inhibitor of mitogen-activated protein kinase (MAP2K1 or MEK), and is currently in pediatric phase II clinical trials for low-grade glioma. We hypothesized that binimetinib would inhibit growth of AT/RT cells by suppressing the MAP kinase pathway. Binimetinib inhibited AT/RT growth at nanomolar concentrations. Binimetinib decreased cell proliferation and induced apoptosis in AT/RT cells and significantly reduced AT/RT tumor growth in flank xenografts. Our data suggest that MAP kinase pathway inhibition could offer a potential avenue for treating these highly aggressive tumors.

Metastasis-directed therapy for oligometastasis and beyond

Metastasis-directed therapy (MDT)-local therapy that is intended to eradicate specific metastatic lesions-has hitherto been used with varying degrees of clinical efficacy and acceptance as a meaningful therapy for metastatic disease. Over the past 25 years, however, the momentum for using MDT to manage patients with metastatic solid tumours has increased, driven by several factors. Among these factors is the recognition that patients with limited metastatic burden could potentially derive survival benefits from MDT. Furthermore, although current systemic therapies are increasingly effective, they are infrequently curative. In addition, technological advances have broadened the spectrum of metastatic lesions that can be treated with ablative intent. Here we aim to briefly review the status of evidence for the clinical benefit of MDT based on current data mainly from trials in patients with oligometastatic disease, discuss the myriad of clinical states that might fall under and beyond the definition of oligometastasis, review technological advances in MDT and their applications beyond oligometastasis, and discuss the need for the continued co-evolution of MDT and systemic therapy as we seek to understand which patients with metastatic cancer can achieve durable remission and how to optimally manage those who cannot.

Emerging and investigational targeted chemotherapy and immunotherapy agents for metastatic brain tumors

INTRODUCTION

Metastases to the central nervous system are the most common cause of malignant intracranial tumors in adults. Current standard of care includes surgery and radiation, but overall survival remains poor. A range of systemic therapies are emerging as promising treatment options for these patients.

AREAS COVERED

This study reviews novel drug regimens that are under investigation in phase 1 and 2 clinical trials. To identify relevant therapies under clinical investigation, a search was performed on http://clinicaltrials.gov and Pubmed with the keywords brain metastasis, Phase I clinical trial, and Phase II clinical trial from 2016 to 2020. The authors detail the mechanisms of action of all trial agents, outline evidence for their utility, and summarize the current state of the field.

EXPERT OPINION

Current advancements in the medical management of brain metastases can be categorized into targeted therapies, methods of overcoming treatment resistance, novel combinations of therapies, and modulation of the tumor microenvironment with a specific focus on immunotherapy. Each of these realms holds great promise for the field going forward. A more streamlined structure for enrollment into clinical trials will be a crucial step in accelerating progress in this area.

Clinical characteristics and treatment outcomes of pigmented tumors in central nervous system: Focusing on melanocytic tumors

Pigmented tumors are rare neoplasm of central nervous system. Melanocytic tumor, including primary and metastatic lesions, is the most common type. Owing to the rarity, the differential diagnosis of pigmented tumors and clinical management of melanocytic tumor remain challenge. Therefore, focusing on melanocytic tumors, the clinical, radiological, histopathological features and treatment outcomes were presented and analyzed in this study. We identified 22 melanocytic tumors, 2 melanotic medulloblastomas, 2 melanotic ependymomas and 1 melanotic schwannoma. Compared with metastatic melanocytic tumors (MMTs), primary melanocytic tumors (PMTs) were characterized by younger age (36.11 ± 17.96 vs. 51.69 ± 12.58 years, p = 0.0262), lower possibility to be multiple lesions (11.1%vs. 61.5%, p = 0.0306), higher proportion of hypointensity on T2-weighted images (66.7% vs. 15.4%, p = 0.0260) and higher frequency in black appearance (77.8% vs. 23.1%, p = 0.0247). During the follow-up, 4 PMTs and 11 MMTs (71.4%) experienced tumor progression. PMTs had better prognosis than MMTs that progression-free survival (PFS) rate of PMT was 50.0% but decreased to 23.1% for MMTs at 12 months (p = 0.0123). Cox proportional hazards regression revealed that multiplicity of tumor was an independent predictor for PFS. None of patient with multiple tumors was in PFS after 12 months' follow-up whereas PFS rate was 40.5% for single tumor (p = 0.0002). In conclusion, radiological appearances, especially hypointensity on T2-weighted images, might be an indication for PMT. MMTs are more likely to be multiple intraparenchymal masses in elder patients located in supratentorial region. Current treatments included operation, radiotherapy and chemotherapy are not competent to control tumor progression and other therapeutic modalities are urgently needed.

Management of brain metastases according to molecular subtypes

The incidence of brain metastases has markedly increased in the past 20 years owing to progress in the treatment of malignant solid tumours, earlier diagnosis by MRI and an ageing population. Although local therapies remain the mainstay of treatment for many patients with brain metastases, a growing number of systemic options are now available and/or are under active investigation. HER2-targeted therapies (lapatinib, neratinib, tucatinib and trastuzumab emtansine), alone or in combination, yield a number of intracranial responses in patients with HER2-positive breast cancer brain metastases. New inhibitors are being investigated in brain metastases from ER-positive or triple-negative breast cancer. Several generations of EGFR and ALK inhibitors have shown activity on brain metastases from EGFR and ALK mutant non-small-cell lung cancer. Immune-checkpoint inhibitors (ICIs) hold promise in patients with non-small-cell lung cancer without druggable mutations and in patients with triple-negative breast cancer. The survival of patients with brain metastases from melanoma has substantially improved after the advent of BRAF inhibitors and ICIs (ipilimumab, nivolumab and pembrolizumab). The combination of targeted agents or ICIs with stereotactic radiosurgery could further improve the response rates and survival but the risk of radiation necrosis should be monitored. Advanced neuroimaging and liquid biopsy will hopefully improve response evaluation.

Tumor mutational burden and driver mutations: Characterizing the genomic landscape of pediatric brain tumors

BACKGROUND

Tumor mutational burden (TMB) and driver mutations are potential biomarkers to guide targeted therapy selection. Malignant gliomas with high TMB in children may preferentially benefit from treatment with immune checkpoint inhibitors (ICPIs). Higher TMB may relate to lower incidence of driver mutations, but this relationship has not been studied in pediatric brain tumors.

PROCEDURE

Comprehensive genomic profiling was performed on 723 pediatric (≤21 years) brain tumor samples using DNA extracted from formalin-fixed paraffin-embedded tissue. TMB was calculated as mutations per megabase and categorized as low (0-6), intermediate (6-20), or high (>20). Analysis included 80 clinically relevant driver mutations; genomic alterations known to confer a selective growth advantage.

RESULTS

Of 723 brain tumors, TMB was low in 91.8%, intermediate in 6.1%, and high in 2.1%. In the high TMB cohort, 93% of tumors harbored a driver mutation; 70% and 63% in the intermediate and low TMB cohorts, respectively (P < 0.05). However, when excluding tumor suppressor genes, high TMB tumors had a decreased incidence of driver mutations (P < 0.001). BRAF alterations were not identified in high TMB tumors, but were enriched in low TMB tumors (P < 0.01). Conversely, there was an association between high TMB tumors and TP53 mutations (P < 10^-13 ). Of the 15 tumors with high TMB, 14 were high-grade gliomas and 13 had alterations in TP53. Three homozygous mismatch repair deletions identified were associated with a higher TMB (P < 0.01).

CONCLUSIONS

Specific driver mutations appear to have a relationship with TMB. These represent populations in which ICPIs may be more or less effective.

Mutant ACVR1 Arrests Glial Cell Differentiation to Drive Tumorigenesis in Pediatric Gliomas

Diffuse intrinsic pontine gliomas (DIPGs) are aggressive pediatric brain tumors for which there is currently no effective treatment. Some of these tumors combine gain-of-function mutations in ACVR1, PIK3CA, and histone H3-encoding genes. The oncogenic mechanisms of action of ACVR1 mutations are currently unknown. Using mouse models, we demonstrate that Acvr1G328V arrests the differentiation of oligodendroglial lineage cells, and cooperates with Hist1h3bK27M and Pik3caH1047R to generate high-grade diffuse gliomas. Mechanistically, Acvr1G328V upregulates transcription factors which control differentiation and DIPG cell fitness. Furthermore, we characterize E6201 as a dual inhibitor of ACVR1 and MEK1/2, and demonstrate its efficacy toward tumor cells in vivo. Collectively, our results describe an oncogenic mechanism of action for ACVR1 mutations, and suggest therapeutic strategies for DIPGs.

Drug Concentration Asymmetry in Tissues and Plasma for Small Molecule-Related Therapeutic Modalities

The well accepted "free drug hypothesis" for small-molecule drugs assumes that only the free (unbound) drug concentration at the therapeutic target can elicit a pharmacologic effect. Unbound (free) drug concentrations in plasma are readily measurable and are often used as surrogates for the drug concentrations at the site of pharmacologic action in pharmacokinetic-pharmacodynamic analysis and clinical dose projection in drug discovery. Furthermore, for permeable compounds at pharmacokinetic steady state, the free drug concentration in tissue is likely a close approximation of that in plasma; however, several factors can create and maintain disequilibrium between the free drug concentration in plasma and tissue, leading to free drug concentration asymmetry. These factors include drug uptake and extrusion mechanisms involving the uptake and efflux drug transporters, intracellular biotransformation of prodrugs, membrane receptor-mediated uptake of antibody-drug conjugates, pH gradients, unique distribution properties (covalent binders, nanoparticles), and local drug delivery (e.g., inhalation). The impact of these factors on the free drug concentrations in tissues can be represented by K _p,uu, the ratio of free drug concentration between tissue and plasma at steady state. This review focuses on situations in which free drug concentrations in tissues may differ from those in plasma (e.g., K _p,uu > or <1) and discusses the limitations of the surrogate approach of using plasma-free drug concentration to predict free drug concentrations in tissue. This is an important consideration for novel therapeutic modalities since systemic exposure as a driver of pharmacologic effects may provide limited value in guiding compound optimization, selection, and advancement. Ultimately, a deeper understanding of the relationship between free drug concentrations in plasma and tissues is needed.