Location of a single histidine within peptide carriers increases mRNA delivery.

BACKGROUND: Previously, we determined that four-branched histidine-lysine (HK) peptides were effective carriers of plasmids and small interfering RNA. In the present study, we compared several branched HK carriers and, in particular, two closely-related H3K4b and H3K(+H)4b peptides for their ability as carriers of mRNA. The H3K(+H)4b peptide differed from its parent analogue, H3K4b, by only a single histidine in each branch. METHODS: A series of four-branched HK peptides with varied sequences was synthesized on a solid-phase peptide synthesizer. The ability of these peptides to carry mRNA expressing luciferase to MDA-MB-231 cells was investigated. With gel retardation and heparin displacement assays, the stability of HK polyplexes was examined. We determined the intracellular uptake of HK polyplexes by flow cytometry and fluorescence microscopy. The size and polydispersity index of the polyplexes in several media were measured by dynamic light scattering. RESULTS: MDA-MB-231 cells transfected by H3K(+H)4b-mRNA polyplexes expressed 10-fold greater levels of luciferase than H3K4b polyplexes. With gel retardation and heparin displacement assays, the H3K(+H)4b polyplexes showed greater stability than H3K4b. Intracellular uptake and co-localization of H3K(+H)4b polyplexes within acidic endosomes were also significantly increased compared to H3K4b. Similar to H3K(+H)4b, several HK analogues with an additional histidine in the second domain of their branches were effective carriers of mRNA. When combined with DOTAP liposomes, H3K(+H)4b was synergistic in delivery of mRNA. CONCLUSIONS: H3K(+H)4b was a more effective carrier of mRNA than H3K4b. Mechanistic studies suggest that H3K(+H)4b polyplexes were more stable than H3K4b polyplexes. Lipopolyplexes formed with H3K(+H)4b markedly increased mRNA transfection.

Identification of a novel differentially methylated region adjacent to ATG16L2 in lung cancer cells using methyl-CpG binding domain protein-enriched genome sequencing.

Lung cancer is the most common cancer worldwide. Epigenetic modifications like DNA methylation play fundamental roles in the dynamic process of lung cancer. The objective of this study was to use methyl-CpG binding domain protein-enriched genome sequencing (MBD-Seq) to identify novel and high-confidence DNA methylation in lung tumor. We first compared the whole-genome DNA methylation of three lung cancer cell lines, including A549, H1299, and SK-MES-1, against BEAS-2B, a lung/bronchial normal epithelial cell line. We then used pyrosequencing and OneStep qMethyl kit methods to verify the results in the cell line specimens. MBD-Seq identified 279, 8046, and 22 887 differentially methylated regions (DMRs), respectively, with 120 common DMRs among three comparison groups. Three DMRs were consistent with the MBD-Seq results by both pyrosequencing and OneStep qMethyl validations. Furthermore, OneStep qMethyl kit was also performed for functional validation of these three potential DMRs in sputum DNA from clinical participants. We successfully identified one new DMR adjacent to ATG16L2. The novel DMR might have an important function in lung carcinogenesis. Further validation of the finding in clinical specimens of lung cancer patients and functional analysis of this novel DMR in the development of lung cancer through transcriptional silencing of ATG16L2 are warranted.

Targeting the KDM4B-AR-c-Myc axis promotes sensitivity to androgen receptor-targeted therapy in advanced prostate cancer.

The histone demethylase KDM4B functions as a key co-activator for the androgen receptor (AR) and plays a vital in multiple cancers through controlling gene expression by epigenetic regulation of H3K9 methylation marks. Constitutively active androgen receptor confers anti-androgen resistance in advanced prostate cancer. However, the role of KDM4B in resistance to next-generation anti-androgens and the mechanisms of KDM4B regulation are poorly defined. Here we found that KDM4B is overexpressed in enzalutamide-resistant prostate cancer cells. Overexpression of KDM4B promoted recruitment of AR to the c-Myc (MYC) gene enhancer and induced H3K9 demethylation, increasing AR-dependent transcription of c-Myc mRNA, which regulates the sensitivity to next-generation AR-targeted therapy. Inhibition of KDM4B significantly inhibited prostate tumor cell growth in xenografts, and improved enzalutamide treatments through suppression of c-Myc. Clinically, KDM4B expression was found upregulated and to correlate with prostate cancer progression and poor prognosis. Our results revealed a novel mechanism of anti-androgen resistance via histone demethylase alteration which could be targeted through inhibition of KDM4B to reduce AR-dependent c-Myc expression and overcome resistance to AR-targeted therapies. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Enhanced tumor uptake and activity of nanoplex-loaded doxorubicin.

Doxorubicin (Dox) has widespread use as a cancer chemotherapeutic agent, but Dox is limited by several side effects including irreversible cardiomyopathy. Although liposomal Dox formulations, such as Doxil, mitigate side effects, they do not prolong survival in many patients. As a result, efforts have continued to discover improved formulations of Dox. We previously found that a peptide-based nanoplex delivered plasmid DNA efficiently to tumors in murine models. Unlike the majority of nanoparticles that depend solely on enhanced permeability and retention (EPR) for their transport into the tumor, our peptide-based nanoplex has a potential advantage in that its uptake primarily depends on neuropilin-1 receptor targeting. Because Dox binds to DNA, we tested whether this delivery platform could effectively deliver Dox to tumors and reduce their size. The nanoplexes increased the levels of Dox in tumors by about 5.5-fold compared to aqueous (free) Dox controls. Consistent with enhanced levels in the tumor, the nanoplex-Dox treatment had significantly greater anti-tumor activity. Whereas low dose free Dox did not reduce the size of tumors compared to untreated controls, the low dose nanoplex-Dox reduced the size of tumors by nearly 55% (p < 0.001). The high dose nanoplex-Dox also inhibited the size of tumor significantly more than the comparable high-dose free Dox (p < 0.001). Furthermore, apoptosis and proliferation markers (Ki67) of tumors observed in the different treatment groups correlated with their ability to inhibit tumor size. This study shows the efficacy of an NRP-1 targeted nanoplexes to deliver Dox to tumors in vivo and lays the groundwork for more complex and effective formulations.

A classifier integrating plasma biomarkers and radiological characteristics for distinguishing malignant from benign pulmonary nodules.

Lung cancer is primarily caused by cigarette smoking and the leading cancer killer in the USA and across the world. Early detection of lung cancer by low-dose CT (LDCT) can reduce the mortality. However, LDCT dramatically increases the number of indeterminate pulmonary nodules (PNs), leading to overdiagnosis. Having a definitive preoperative diagnosis of malignant PNs is clinically important. Using microarray and droplet digital PCR to directly profile plasma miRNA expressions of 135 patients with PNs, we identified 11 plasma miRNAs that displayed a significant difference between patients with malignant versus benign PNs. Using multivariate logistic regression analysis of the molecular results and clinical/radiological characteristics, we developed an integrated classifier comprising two miRNA biomarkers and one radiological characteristic for distinguishing malignant from benign PNs. The classifier had 89.9% sensitivity and 90.9% specificity, being significantly higher compared with the biomarkers or clinical/radiological characteristics alone (all p < 0.05). The classifier was validated in two independent sets of patients. We have for the first time shown that the integration of plasma biomarkers and radiological characteristics could more accurately identify lung cancer among indeterminate PNs. Future use of the classifier could spare individuals with benign growths from the harmful diagnostic procedures, while allowing effective treatments to be immediately initiated for lung cancer, thereby reduces the mortality and cost. Nevertheless, further prospective validation of this classifier is warranted.

NRP1 transport of cancer therapeutics mediated by tumor-penetrating peptides.

Whereas uptake of low molecular weight agents is generally inhibited in tumors due to high interstitial pressure, tumor uptake of macromolecules is increased due to enhanced permeability and retention (EPR). Small molecule drugs alone or incorporated in nanoparticles (NP) have largely been dependent on such physical tumor uptake (passive) for therapeutic activity. Although passive targeted NP such as Stealth Liposomal Doxorubicin (Doxil ®) are effective with improved safety, drug delivery to tumors is still significantly limited. To improve tumor delivery and efficacy, tumor-penetrating peptides (TPP), which contain sequences that target the tumor and activate the neuropilin-1 receptor (NRP1), have either been co-administered with or conjugated to both small and large therapeutic molecules. In this review, we will discuss TPP-mediated therapeutics which target the NRP1 transport system of tumors.

The neuropilin-1 receptor mediates enhanced tumor delivery of H2K polyplexes.

BACKGROUND: Promising plasmid-based treatments have limited value without an effective delivery system. Recently, the linear H2K with a repeating -KHHK- pattern was determined to be an effective plasmid carrier to tumor xenografts in vivo. Although unpacking of the H2K polyplex within the tumor may have a role, the mechanism for the enhanced efficacy remains unclear. METHODS: After solid-phase synthesis of linear and branched histidine-lysine (HK) peptide carriers of plasmids, the peptides were compared for their ability to lyse endosomes with a red blood cell model and to transfect MDA-MB-435 xenografts in the presence or absence of neuropilin-1 receptor (NRP-1) antibodies. To examine stability, polyplexes were incubated with trypsin or NaCl and then analyzed by electrophoresis. RESULTS: After screening peptides with a model for endosomal lysis at two pHs, the 33-mer H3K peptide lysed red blood cells effectively at the lower pH. Combining H3K and H2K peptides as carriers of plasmids expressing luciferase were more effective than H2K alone. Based on the repeating -KHHK- sequences of H2K, we studied whether the widespread gene expression in the tumor may be mediated by NRP-1. By blocking NRP-1 in tumor-bearing mice, luciferase activity in tumors delivered by HK polyplexes was reduced by 96%, whereas activity in normal tissues was minimally reduced. CONCLUSIONS: Combining an endosomolytic peptide, H3K, with H2K polyplexes as a carrier further enhanced transfection in vivo. Moreover, the widespread distribution of H2K polyplexes is mediated by NRP-1, suggesting that transcytosis of these polyplexes through the tumor endothelium may lead to efficient transfection. Copyright © 2016 John Wiley & Sons, Ltd.

Silver adducts of four-branched histidine rich peptides exhibit synergistic antifungal activity.

Previously, a four branched histidine-lysine rich peptide, H3K4b, was shown to demonstrate selective antifungal activity with minimal antibacterial activity. Due to the potential breakdown from proteases, H3K4b was further evaluated in the current study by varying the D- and l-amino acid content in its branches. Whereas analogues of H3K4b that selectively replaced l-amino acids (H3k4b, h3K4b) had improved antifungal activity, the all d-amino acid analogue, h3k4b, had reduced activity, suggesting that partial breakdown of the peptide may be necessary. Moreover, because histidines form coordination bonds with the silver ion, we examined whether silver adducts can be formed with these branched histidine-lysine peptides, which may improve antifungal activity. For Candida albicans, the silver adduct of h3K4b or H3k4b reduced the MIC compared to peptide and silver ions alone by 4- and 5-fold, respectively. For Aspergillus fumigatus, the silver adducts showed even greater enhancement of activity. Although the silver adducts of H3k4b or h3K4b showed synergistic activity, the silver adduct with the all l-amino acid H3K4b surprisingly showed the greatest synergistic and growth inhibition of A. fumigatus: the silver adduct of H3K4b reduced the MIC compared to the peptide and silver ions alone by 30- and 26-fold, respectively. Consistent with these antifungal efficacy results, marked increases in free oxygen radicals were produced with the H3K4b and silver combination. These studies suggest that there is a balance between stability and breakdown for optimal antifungal activity of the peptide alone and for the peptide-silver adduct.

Increased tumor distribution and expression of histidine-rich plasmid polyplexes.

BACKGROUND: Selecting nonviral carriers for in vivo gene delivery is often dependent on determining the optimal carriers from transfection assays in vitro. The rationale behind this in vitro strategy is to cast a net sufficiently wide to identify the few effective carriers of plasmids for in vivo studies. Nevertheless, many effective in vivo carriers may be overlooked by this strategy because of the marked differences between in vitro and in vivo assays. METHODS: After solid-phase synthesis of linear and branched histidine/lysine (HK) peptides, the two peptide carriers were compared for their ability to transfect MDA-MB-435 tumor cells in vitro and then in vivo. RESULTS: By contrast to their transfection activity in vitro, the linear H2K carrier of plasmids was far more effective in vivo compared to the branch H2K4b. Surprisingly, negatively-charged polyplexes formed by the linear H2K peptide gave higher transfection in vivo than did those with a positive surface charge. To examine the distribution of plasmid expression within the tumor from H2K polyplexes, we found widespread expression by immunohistochemical staining. With a fluorescent tdTomato expressing-plasmid, we confirmed a pervasive distribution and gene expression within the tumor mediated by the H2K polyplex. CONCLUSIONS: Although mechanisms underlying the efficiency of gene expression are probably multifactorial, unpacking of the H2K polyplex within the tumor appears to have a significant role. Further development of these H2K polyplexes represents an attractive approach for plasmid-based therapies of cancer.

Direct observation of dynamic mechanical regulation of DNA condensation by environmental stimuli.

Gene delivery is a promising way to treat hereditary diseases and cancer; however, there is little understanding of DNA:carrier complex mechanical properties, which may be critical for the protection and release of nucleic acids. We applied optical tweezers to directly measure single-molecule mechanical properties of DNA condensed using 19-mer poly-L-lysine (PLL) or branched histidine-lysine (HK) peptides. Force-extension profiles indicate that both carriers condense DNA actively, showing force plateaus during stretching and relaxation cycles. As the environment such as carrier concentration, pH, and the presence of zinc ions changes, DNA:HK complexes showed dynamically regulated mechanical properties at multiple force levels. The fundamental knowledge from this study can be applied to design a mechanically tailored complex which may enhance transfection efficiency by controlling the stability of the complex temporally and spatially.

Cytokine Signatures for Lung Cancer Diagnosis in African American Populations.

Lung cancer is the leading cause of cancer-related deaths among both men and women. African Americans (AAs) experience disproportionately higher incidence and mortality compared to other ethnic groups. Cytokines play multifaceted and crucial roles in the initiation, progression, and spread of cancer. Our aim was to identify cytokine biomarkers for the early detection of lung cancer in AAs. We examined eight key cytokines (Interleukin-1, IL-6, IL-8, IL-10, IL-12p70, monocyte chemotactic protein-1 (MCP-1), interferon-gamma (IFN-γ), and tumor necrosis factor-alpha (TNF-α)) in the plasma of 104 lung cancer patients and 48 cancer-free individuals using the FirePlex Immunoassay. These findings were subsequently validated in a separate cohort of 58 cases and 58 controls. IL-8, IFN-γ, and TNF-α exhibited elevated levels in both AA and White American (WA) lung cancer cases. Notably, IL-10 and MCP-1 displayed significant increases specifically in AA lung cancer patients, with MCP-1 levels associated with lung adenocarcinoma cases. Conversely, WA lung cancer patients showed heightened IL-6 levels, particularly linked to lung adenocarcinoma. The combined use of specific cytokines showed promise in lung cancer diagnosis, with IL-8, IL-10, and MCP-1 achieving 76% sensitivity and 79% specificity in AAs and IL-6 and IL-8 combined offering 76% sensitivity and 74% specificity in WAs. These diagnostic biomarkers were validated in the independent cohort. The ethnicity-related cytokine biomarkers hold promise for diagnosing lung cancer in AAs and WAs, potentially addressing the observed racial disparity.

Delivery of mRNA with Histidine-Lysine Peptides.

Transfection with mRNA has been considered superior to that with plasmids since the mRNA can be translated to a protein in the cytosol without entering the nucleus. One disadvantage of using mRNA is its susceptibility to enzymatic biodegradability, and consequently, significant research has occurred to determine nonviral carriers that will sufficiently stabilize this nucleic acid for cellular transport. Histidine-lysine peptides (HK) are one such class of mRNA carriers, which we think serves as a model for other peptides and polymeric carrier systems. When the HK peptide and mRNA are mixed and interact through ionic and nonionic bonds, mRNA polyplexes are formed, which can transfect cells. In contrast to linear HK peptides, branched HK peptides protected and efficiently transfected mRNA into cells. After describing the preparation and biophysical characterization of these polyplexes, we will provide protocols for in vitro and in vivo transfection for these mRNA polyplexes.

pH-Dependent Non-Covalent Release of Chemotherapy from Carriers.

Although Warburg discovered pH discrepancies between tumor and normal tissues nearly 100 years ago, developing therapies to take advantage of this concept was relatively slow for the first 70 years. During the last 30 years, there has been an exponential increase in the use of pH-dependent strategies for both low molecular weight drugs and nanoparticles. Two frequently discussed approaches are the chemotherapy's release from pH-sensitive covalent linkages of macromolecules or from pH-dependent disruption of charged polymeric nanoparticles. In contrast, pH-dependent non-covalent bonds between the chemotherapy agent and macromolecules have rarely been discussed, yet this underappreciated strategy has great potential. These non-covalent interactions are primarily ionic or hydrogen bonds with supporting roles from hydrophobic bonds. In addition to the facile coupling of the drug with the carrier, these non-covalent interactions may show marked pH dependence. Consistent with pH dependence, many of these drug-loaded carriers showed significant in vitro and, in some cases, striking in vivo activity. In this review, we will focus on pH-sensitive non-covalent bonds, highlighting the release of drugs from diverse carriers such as tetrahedron DNA structures, cyclodextrin, polymeric carriers, and carbon-based quantum particles.

Surface-modified HK:siRNA nanoplexes with enhanced pharmacokinetics and tumor growth inhibition.

We characterized in this study the pharmacokinetics and antitumor efficacy of histidine-lysine (HK):siRNA nanoplexes modified with PEG and a cyclic RGD (cRGD) ligand targeting αvß3 and αvß5 integrins. With noninvasive imaging, systemically administered surface-modified HK:siRNA nanoplexes showed nearly 4-fold greater blood levels, 40% higher accumulation in tumor tissue, and 60% lower luciferase activity than unmodified HK:siRNA nanoplexes. We then determined whether the surface-modified HK:siRNA nanoplex carrier was more effective in reducing MDA-MB-435 tumor growth with an siRNA targeting Raf-1. Repeated systemic administration of the selected surface modified HK:siRNA nanoplexes targeting Raf-1 showed 35% greater inhibition of tumor growth than unmodified HK:siRNA nanoplexes and 60% greater inhibition of tumor growth than untreated mice. The improved blood pharmacokinetic results and tumor localization observed with the integrin-targeting surface modification of HK:siRNA nanoplexes correlated with greater tumor growth inhibition. This investigation reveals that through control of targeting ligand surface display in association with a steric PEG layer, modified HK: siRNA nanoplexes show promise to advance RNAi therapeutics in oncology and potentially other critical diseases.

Buffering capacity and size of siRNA polyplexes influence cytokine levels.

Induction of cytokines by small interfering RNA (siRNA) polyplexes has been a significant concern of researchers attempting to minimize the toxicity of this promising therapy. Although cationic carriers of siRNA are known to increase cytokine levels, few systematic studies have been done to determine what properties of the carrier are important to modulate cytokines. Because branched histidine-lysine (HK) peptides are effective carriers of siRNA and their sequence can be readily modified, we selected this class of carrier to determine which sequences of the peptide were important for cytokine induction. With the use of peripheral blood mononuclear cells (PBMCs), the HK peptide with a higher number of histidines (H3K(+H)4b) in complex with siRNA induced lower levels of cytokines compared with other HK (e.g., H2K4b, H3K4b, H3K(+N)4b) siRNA nanoplexes. Notably, these peptides' siRNA polyplexes showed a similar pattern of cytokine induction when injected intravenously in a mouse model, i.e., the HK with higher content of histidines induced cytokines the least. As indicated by the pH-sensitive dye within acidic endosomes, the greater pH-buffering capacity of H3K(+H)4b compared with other HK peptides may explain why cytokine levels were reduced. In addition to buffering capacity, the size of HK polyplexes markedly influenced cytokine production.

Circulating Bacterial DNA as Plasma Biomarkers for Lung Cancer Early Detection.

Lung cancer is a leading cause of cancer deaths and early diagnosis can significantly improve outcomes. Pathogenic bacteria have been shown to play a role in tumorigenesis and its analysis provides a new approach for cancer diagnosis. To evaluate the potential of bacteria as plasma biomarkers for early lung cancer detection, we analyzed eight lung-cancer-related bacterial genera in 58 lung cancer patients and 58 controls using ddPCR. Our results showed that five genera had higher DNA abundance in lung tumor tissues compared with normal tissues. Three of these genera (Selenomonas, Streptococcus, and Veillonella) displayed consistent changes in plasma, with higher DNA abundance in lung cancer patients compared with controls. When used as a panel, these three bacterial genera had a sensitivity of 75% and specificity of 78% for lung cancer detection, regardless of stage or histology. The performance of this biomarker panel was confirmed in an independent cohort of 93 lung cancer cases and 93 controls. Thus, circulating bacterial DNA has the potential to be used as plasma biomarkers for early lung cancer detection.

Delivery of Chemotherapy Agents and Nucleic Acids with pH-Dependent Nanoparticles.

With less than one percent of systemically injected nanoparticles accumulating in tumors, several novel approaches have been spurred to direct and release the therapy in or near tumors. One such approach depends on the acidic pH of the extracellular matrix and endosomes of the tumor. With an average pH of 6.8, the extracellular tumor matrix provides a gradient for pH-responsive particles to accumulate, enabling greater specificity. Upon uptake by tumor cells, nanoparticles are further exposed to lower pHs, reaching a pH of 5 in late endosomes. Based on these two acidic environments in the tumor, various pH-dependent targeting strategies have been employed to release chemotherapy or the combination of chemotherapy and nucleic acids from macromolecules such as the keratin protein or polymeric nanoparticles. We will review these release strategies, including pH-sensitive linkages between the carrier and hydrophobic chemotherapy agent, the protonation and disruption of polymeric nanoparticles, an amalgam of these first two approaches, and the release of polymers shielding drug-loaded nanoparticles. While several pH-sensitive strategies have demonstrated marked antitumor efficacy in preclinical trials, many studies are early in their development with several obstacles that may limit their clinical use.

Zinc Finger Nucleases: Tailor-made for Gene Therapy.

Genome editing with the use of zinc finger nucleases has been successfully applied to variety of a eukaryotic cells. Furthermore, the proof of concept for this approach has been extended to diverse animal models from Drosophila to mice. Engineered zinc finger nucleases are able to target specifically and manipulate disease-causing genes through site-specific double strand DNA breaks followed by non-homologous end joining or homologous recombination mechanisms. Consequently, this technology has considerable flexibility that can result in either a gain or loss of function of the targeted gene. In addition to this flexibility, gene therapy by zinc finger nucleases may enable persistent long term gene modification without continuous transfection- a potential advantage over RNA interference or direct gene inhibitors. With systemic viral delivery systems, this gene-editing approach corrected the mutant factor IX in models of mouse hemophilia. Moreover, phase I clinical trials have been initiated with zinc finger nucleases in patients with glioblastoma and HIV. Thus, this emerging field has significant promise as a therapeutic strategy for human genetic diseases, infectious diseases and oncology. In this article, we will review recent advances and potential risks in zinc finger nuclease gene therapy.

Novel Method for Rapid Detection of COVID-19 Omicron Variant and Future Emerging Variants.

Over six million people have died worldwide as a result of SARS-CoV-19 (Covid-19). Several major and minor variant waves of Covid-19 including the Alpha (B.1.1.7), Delta (B.1.617.2), and the Omicron (B.1.1.529) have infected patients in the last two years. These individual variants have been associated with different infectivity and death rates, and it is anticipated that other variants of significance will occur. As a result, the emergence and frequency of the different Covid-19 variants at various locations in the world are critically important to monitor. To date, sequencing of these variants has been the primary method to track the prevalence of the different variants, but this approach is relatively slow and expensive. In contrast, we have developed two qPCR methods that are sensitive, specific, and economical which can distinguish the Omicron variant from the other variants. The first qPCR method detects a specific base mutation in Omicron enabling improved amplification. The second method is based on designing a primer specific for the region in which a deletion and an insertion have occurred in the Omicron variant. While the first approach can be readily adapted to identify other variants with point mutations, the second method can identify emerging deletion/insertion mutations such as BA.2, a subvariant of the Omicron.

pH-Sensitive Targeting of Tumors with Chemotherapy-Laden Nanoparticles: Progress and Challenges.

Accumulating chemotherapeutic drugs such as doxorubicin within a tumor while limiting the drug dose to normal tissues is a central goal of drug delivery with nanoparticles. Liposomal products such as Doxil® represent one of the marked successes of nanoparticle-based strategies. To replicate this success for cancer treatment, many approaches with nanoparticles are being explored in order to direct and release chemotherapeutic agents to achieve higher accumulation in tumors. A promising approach has been stimulus-based therapy, such as the release of chemotherapeutic agents from the nanoparticles in the acidic environments of the tumor matrix or the tumor endosomes. Upon reaching the acidic environments of the tumor, the particles, which are made up of pH-dependent polymers, become charged and release the entrapped chemotherapy agents. This review discusses recent advances in and prospects for pH-dependent histidine-based nanoparticles that deliver chemotherapeutic agents to tumors. The strategies used by investigators include an array of histidine-containing peptides and polymers which form micelles, mixed micelles, nanovesicles, polyplexes, and coat particles. To date, several promising histidine-based nanoparticles have been demonstrated to produce marked inhibition of tumor growth, but challenges remain for successful outcomes in clinical trials. The lessons learned from these histidine-containing particles will provide insight in the development of improved pH-dependent polymeric delivery systems for chemotherapy.