An effective cell-penetrating antibody delivery platform

Engineering IgG antibodies for intracellular targeting and drug delivery

Enabling immunoglobulin G (IgG)-format antibodies to autonomously internalize and localize in the cytosol of targeted cells-referred to as cytosol-penetrating antibodies (cytotransmab, CT)-is challenging yet highly promising. A primary barrier to cytosolic access for CT is limited endosomal escape. Herein, we developed a second-generation (2G) CT, named in2CT4.1, featuring an endosomal acidic pH-responsive endosomal escape motif (R-W/E motif) with Arg-Trp pairs and a Glu patch in the CH3 and CL domains of IgG1/κ antibody. This motif selectively destabilizes endosomal membranes at endosomal acidic pH to facilitate cytosolic access while remaining inactive at neutral pH. The 2G CT, in2CT4.1, achieves efficient cytosolic localization at nanomolar concentrations, demonstrating approximately 3-fold higher endosomal escape efficiency compared to the first-generation CT. The potential of 2G CT is validated by engineering a cytosolic α-tubulin-targeting CT via an α-tubulin-specific variable domain in in2CT4.1. Additionally, the 2G CT effectively delivers the catalytic domain of diphtheria toxin to the cytosol of epidermal growth factor receptor-overexpressing tumor cells, resulting in near-complete suppression of tumor growth in a xenograft mouse model. These results establish 2G CT as a versatile platform for targeting cytosolic proteins and delivering therapeutic payloads, with broad potential in targeted cancer therapy and other applications.

A Critical Role of Intracellular PD-L1 in Promoting Ovarian Cancer Progression

Disrupting the interaction between tumor-cell surface PD-L1 and T cell membrane PD-1 can elicit durable clinical responses. However, only about 10% of ovarian cancer patients respond to PD-1/PD-L1 blockade. Here, we show that PD-L1 expression in ovarian cancer-patient tumors is predominantly intracellular. Notably, PARP inhibitor treatment highly increased intracellular PD-L1 accumulation in both ovarian cancer-patient tumor samples and cell lines. We investigated whether intracellular PD-L1 might play a critical role in ovarian cancer progression. Mutating the PD-L1 acetylation site in PEO1 and ID8Brca1-/- ovarian cancer cells significantly decreased PD-L1 levels and impaired colony formation, which was accompanied by cell cycle G2/M arrest and apoptosis induction. PEO1 and ID8Brca1-/- tumors with PD-L1 acetylation site mutation also exhibited significantly reduced growth in mice. Furthermore, targeting intracellular PD-L1 with a cell-penetrating antibody effectively decreased ovarian tumor-cell intracellular PD-L1 level and induced tumor-cell growth arrest and apoptosis, as well as enhanced DNA damage and STING activation, both in vitro and in vivo. In conclusion, we have shown the critical role of intracellular PD-L1 in ovarian cancer progression.

A platform to deliver single and bi-specific Cas9/guide RNA to perturb genes in vitro and in vivo

Although CRISPR-Cas9 technology is poised to revolutionize the treatment of diseases with underlying genetic mutations, it faces some significant issues limiting clinical entry. They include low-efficiency in vivo systemic delivery and undesired off-target effects. Here, we demonstrate, by modifying Cas9 with phosphorothioate-DNA oligos (PSs), that one can efficiently deliver single and bi-specific CRISPR-Cas9/guide RNA (gRNA) dimers in vitro and in vivo with reduced off-target effects. We show that PS-Cas9/gRNA-mediated gene knockout preserves chimeric antigen receptor T cell viability and expansion in vitro and in vivo. PS-Cas9/gRNA mediates gene perturbation in patient-derived tumor organoids and mouse xenograft tumors, leading to potent tumor antitumor effects. Further, HER2 antibody-PS-Cas9/gRNA conjugate selectively perturbs targeted genes in HER2+ ovarian cancer xenografts in vivo. Moreover, we created bi-specific PS-Cas9 with two gRNAs to target two adjacent sequences of the same gene, leading to efficient targeted gene disruption ex vivo and in vivo with markedly reduced unintended gene perturbation. Thus, the cell-penetrating PS-Cas9/gRNA can achieve efficient systemic delivery and precision in gene disruption.

Recent approaches of antibody therapeutics in androgenetic alopecia

Therapeutic antibodies (Abs) have been anticipated as promising alternatives to conventional treatments such as topical minoxidil and oral finasteride for androgenetic alopecia (AGA). Due to the high molecular weight of typical Abs, the half-life of subcutaneous Abs exceeds 2 weeks, allowing an administration intervals of once a month or longer. Direct injection into the areas of hair loss is also feasible, potentially enhancing treatment efficacy while minimizing systemic side effects. However, therapeutic Abs are rarely developed for AGA therapy due to the requirement to be responsiveness to androgens and to exist in the extracellular fluid or cell surface surrounding the hair follicle. In this review, we introduce recent progress of antibody therapeutics in AGA targeting the prolactin receptor, Interleukin-6 receptor, C-X-C motif chemokine ligand 12, and dickkopf 1. As therapeutic Abs for AGA are still in the early stages, targets need further validation and optimization for clinical application.

Rescue of expression and function of long QT syndrome-causing mutant hERG channels by enhancing channel stability in the plasma membrane

The human ether-a-go-go-related gene (hERG) encodes the Kv11.1 (or hERG) channel that conducts the rapidly activating delayed rectifier potassium current (IKr). Naturally occurring mutations in hERG impair the channel function and cause long QT syndrome type 2. Many missense hERG mutations lead to a lack of channel expression on the cell surface, representing a major mechanism for the loss-of-function of mutant channels. While it is generally thought that a trafficking defect underlies the lack of channel expression on the cell surface, in the present study, we demonstrate that the trafficking defective mutant hERG G601S can reach the plasma membrane but is unstable and quickly degrades, which is akin to WT hERG channels under low K+ conditions. We previously showed that serine (S) residue at 624 in the innermost position of the selectivity filter of hERG is involved in hERG membrane stability such that substitution of serine 624 with threonine (S624T) enhances hERG stability and renders hERG insensitive to low K+ culture. Here, we report that the intragenic addition of S624T substitution to trafficking defective hERG mutants G601S, N470D, and P596R led to a complete rescue of the function of these otherwise loss-of-function mutant channels to a level similar to the WT channel, representing the most effective rescue means for the function of mutant hERG channels. These findings not only provide novel insights into hERG mutation-mediated channel dysfunction but also point to the critical role of S624 in hERG stability on the plasma membrane.

Molecular and therapeutic advancements in Capicua (CIC)-rearranged sarcoma

Capicua (CIC)-rearranged sarcomas are an aggressive subset of undifferentiated round cell sarcomas. CIC::DUX4, the proto-typical CIC fusion oncoprotein is associated with rapid clinical progression and chemotherapy resistance leading to poor clinical outcomes. Recent studies have identified additional CIC fusions (CIC::NUTM1, CIC::FOXO4, and CIC::LEUTX) that largely retain CIC-binding specificity but leverage C-terminal binding partners (NUTM1, FOXO4, and LEUTX) to potentially activate transcriptional programs that drive oncogenesis. Moreover, the recent development of preclinical models to study CIC::DUX4 sarcoma have advanced our understanding of the underlying biological mechanisms and uncovered key dependencies that can be translated into rational therapies. In this review, we will highlight these recent advancements in CIC-rearranged sarcoma biology with a vision for clinical translation to improve patient outcomes.

Pin1-Catalyzed Conformation Changes Regulate Protein Ubiquitination and Degradation

The unique prolyl isomerase Pin1 binds to and catalyzes cis-trans conformational changes of specific Ser/Thr-Pro motifs after phosphorylation, thereby playing a pivotal role in regulating the structure and function of its protein substrates. In particular, Pin1 activity regulates the affinity of a substrate for E3 ubiquitin ligases, thereby modulating the turnover of a subset of proteins and coordinating their activities after phosphorylation in both physiological and disease states. In this review, we highlight recent advancements in Pin1-regulated ubiquitination in the context of cancer and neurodegenerative disease. Specifically, Pin1 promotes cancer progression by increasing the stabilities of numerous oncoproteins and decreasing the stabilities of many tumor suppressors. Meanwhile, Pin1 plays a critical role in different neurodegenerative disorders via the regulation of protein turnover. Finally, we propose a novel therapeutic approach wherein the ubiquitin-proteasome system can be leveraged for therapy by targeting pathogenic intracellular targets for TRIM21-dependent degradation using stereospecific antibodies.

Targeting PARG induces tumor cell growth inhibition and antitumor immune response by reducing phosphorylated STAT3 in ovarian cancer

BACKGROUND

Ovarian cancer is the most lethal gynecological malignancy, with limited treatment options after failure of standard therapies. Despite the potential of poly(ADP-ribose) polymerase inhibitors in treating DNA damage response (DDR)-deficient ovarian cancer, the development of resistance and immunosuppression limit their efficacy, necessitating alternative therapeutic strategies. Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) represent a novel class of inhibitors that are currently being assessed in preclinical and clinical studies for cancer treatment.

METHODS

By using a PARG small-molecule inhibitor, COH34, and a cell-penetrating antibody targeting the PARG's catalytic domain, we investigated the effects of PARG inhibition on signal transducer and activator of transcription 3 (STAT3) in OVCAR8, PEO1, and Brca1-null ID8 ovarian cancer cell lines, as well as in immune cells. We examined PARG inhibition-induced effects on STAT3 phosphorylation, nuclear localization, target gene expression, and antitumor immune responses in vitro, in patient-derived tumor organoids, and in an immunocompetent Brca1-null ID8 ovarian mouse tumor model that mirrors DDR-deficient human high-grade serous ovarian cancer. We also tested the effects of overexpressing a constitutively activated STAT3 mutant on COH34-induced tumor cell growth inhibition.

RESULTS

Our findings show that PARG inhibition downregulates STAT3 activity through dephosphorylation in ovarian cancer cells. Importantly, overexpression of a constitutively activated STAT3 mutant in tumor cells attenuates PARG inhibitor-induced growth inhibition. Additionally, PARG inhibition reduces STAT3 phosphorylation in immune cells, leading to the activation of antitumor immune responses, shown in immune cells cocultured with ovarian cancer patient tumor-derived organoids and in immune-competent mice-bearing mouse ovarian tumors.

CONCLUSIONS

We have identified a novel antitumor mechanism underlying PARG inhibition beyond its primary antitumor effects through blocking DDR in ovarian cancer. Furthermore, targeting PARG activates antitumor immune responses, thereby potentially increasing response rates to immunotherapy in patients with ovarian cancer.

ACE2 Receptor-Targeted Inhaled Nanoemulsions Inhibit SARS-CoV-2 and Attenuate Inflammatory Responses

Three kinds of coronaviruses are highly pathogenic to humans, and two of them mainly infect humans through Angiotensin-converting enzyme 2 （ACE2）receptors. Therefore, specifically blocking ACE2 binding at the interface with the receptor-binding domain is promising to achieve both preventive and therapeutic effects of coronaviruses. Alternatively, drug-targeted delivery based on ACE2 receptors can further improve the efficacy and safety of inhalation drugs. Here, these two approaches are innovatively combined by designing a nanoemulsion (NE) drug delivery system (termed NE-AYQ) for inhalation that targets binding to ACE2 receptors. This inhalation-delivered remdesivir nanoemulsion (termed RDSV-NE-AYQ) effectively inhibits the infection of target cells by both wild-type and mutant viruses. The RDSV-NE-AYQ strongly inhibits Severe acute respiratory syndrome coronavirus 2 at two dimensions: they not only block the binding of the virus to host cells at the cell surface but also restrict virus replication intracellularly. Furthermore, in the mouse model of acute lung injury, the inhaled drug delivery system loaded with anti-inflammatory drugs (TPCA-1-NE-AYQ) can significantly alleviate the lung tissue injury of mice. This smart combination provides a new choice for dealing with possible emergencies in the future and for the rapid development of inhaled drugs for the treatment of respiratory diseases.

Functional antibody delivery: Advances in cellular manipulation

The current therapeutic antibody market in the U.S. consists of 100 antibody-based products and their market value is expected to explode beyond $300 billion by 2025. These therapies are presently limited to extracellular targets due to the innate inability of antibodies to transverse membranes. To expand the number of accessible therapeutic targets, intracellular antibody delivery is necessary. Many delivery vehicles for antibodies have been used with some promising results, such as nanoparticles and cell penetrating polymers. Despite the success of these delivery platforms using model antibody cargo, there is a surprisingly small number of studies that focus on functional antibody delivery into the cytosol that also measures a cellular response. Antibodies can be designed for essentially unlimited targets, including proteins and DNA, that will ultimately control cell function once delivered inside cells. Advancement in cellular manipulation depends on the application of intracellularly delivering functional antibodies to achieve a desired result. This review focuses on the emerging field of functional antibody delivery which enables various cellular responses and cell manipulation.

Post-genomic platform for development of oligonucleotide vaccines against RNA viruses: diamond cuts diamond

The coronavirus pandemic has starkly demonstrated the need to create highly effective vaccines against various viral diseases. The emerging new platforms for vaccine creation (adenovirus vectors and mRNA vaccines) have shown their worth in the fight against the prevention of coronavirus infection. However, adenovirus vectors and mRNA vaccines have a serious disadvantage: as a rule, only the S protein of the coronavirus is presented as an antigen. This tactic for preventing infection allows the ever-mutating virus to escape quickly from the immunity protection provided by such vaccines. Today, viral genomic databases are well-developed, which makes it possible to create new vaccines on a fundamentally new post-genomic platform. In addition, the technology for the synthesis of nucleic acids is currently experiencing an upsurge in demand in various fields of molecular biology. The accumulated experience suggests that the unique genomic sequences of viruses can act as antigens that trigger powerful humoral and cellular immunity. To achieve this effect, the following conditions must be created: the structure of the nucleic acid must be single-stranded, have a permanent 3D nanostructure, and have a unique sequence absent in the vaccinated organism. Oligonucleotide vaccines are able to resist the rapidly changing genomic sequences of RNA viruses by using conserved regions of their genomes to generate a long-term immune response, acting according to the adage that a diamond cuts a diamond. In addition, oligonucleotide vaccines will not contribute to antibody-dependent enhanced infection, since the nucleic acid of the coronavirus is inside the viral particle. It is obvious that new epidemics and pandemics caused by RNA viruses will continue to arise periodically in the human population. The creation of new, safe, and effective platforms for the production of vaccines that can flexibly change and adapt to new subtypes of viruses is very urgent and at this moment should be considered as a strategically necessary task.

The role of cell-penetrating peptides in potential anti-cancer therapy

Due to the complex physiological structure, microenvironment and multiple physiological barriers, traditional anti-cancer drugs are severely restricted from reaching the tumour site. Cell-penetrating peptides (CPPs) are typically made up of 5-30 amino acids, and can be utilised as molecular transporters to facilitate the passage of therapeutic drugs across physiological barriers. Up to now, CPPs have widely been used in many anti-cancer treatment strategies, serving as an excellent potential choice for oncology treatment. However, their drawbacks, such as the lack of cell specificity, short duration of action, poor stability in vivo, compatibility problems (i.e. immunogenicity), poor therapeutic efficacy and formation of unwanted metabolites, have limited their further application in cancer treatment. The cellular uptake mechanisms of CPPs involve mainly endocytosis and direct penetration, but still remain highly controversial in academia. The CPPs-based drug delivery strategy could be improved by clever design or chemical modifications to develop the next-generation CPPs with enhanced cell penetration capability, stability and selectivity. In addition, some recent advances in targeted cell penetration that involve CPPs provide some new ideas to optimise CPPs.

Reversible Chemical Protein Modification to Endogenous Glutathione and Its Utilities in the Manufacture of Transcellular Pro-Enzymes

Proteins have been perceived as being an intriguing modality of therapeutics for the treatment of intractable diseases in view of their superlative precision and versatility. Nonetheless, proteins' intrinsic characters, particularly their being hydrophilic macromolecules with unmethodical charges, have imposed the exceeding challenge of seeking transcellular trafficking into cells' interiors. To circumvent this drawback, we have attempted to employ triple-functional amine-reactive 4-(2-((2-(((4-nitrophenoxy)carbonyl)oxy)ethyl)disulfaneyl)ethoxy)-4-oxobutanoic acid for the efficient incorporation of the anionic carboxyl moiety into amine-enriched enzymes, resulting in overall negatively charged pro-enzymes. The resulting pro-enzymes could be readily electrostatically assembled with cationic species [for instance: block copolymers of poly(ethylene glycol)-polylysine] into core-shell architectural delivery nanoparticles for their facilitated endocytosis into cells. Noteworthy is the aforementioned carboxylation chemistry designed to allow facile reversal of the pro-enzymes to the original amine groups due to the thiolysis of intermediate disulfide linkage for subsequent cascade reactions in response to the cytosol-enriched glutathione. Therefore, cytosol-selective structural disassembly for the liberation and activation of the pro-enzymes was accomplished. Our subsequent investigations utilizing ribonuclease A and catalase as the model enzymes demonstrated appreciable transcellular transportation of the active enzymes to the cell interiors, exerting overwhelming cytotoxic potencies and H₂O₂ scavenging capacities, respectively. Hence, we reported an unprecedented redox-stimulated charge reversal strategy in engineering cytosol-activatable pro-enzymes, manifesting a simple and efficient approach in the manufacture of transcellular proteinic therapeutics, which should be highlighted to promote their wide availability for use with diverse functional proteins as molecular biological tools and precision therapeutics.

Targeted protein degradation using intracellular antibodies and its application to neurodegenerative disease

Antibodies mediate the majority of their effects in the extracellular domain, or in intracellular compartments isolated from the cytosol. Under a growing list of circumstances, however, antibodies are found to gain access to the cytoplasm. Cytosolic immune complexes are bound by the atypical antibody receptor TRIM21, which mediates the rapid degradation of the immune complexes at the proteasome. These discoveries have informed the development of TRIM-Away, a technique to selectively deplete proteins using delivery of antibodies into cells. A range of related approaches that elicit selective protein degradation using intracellular constructs linking antibody fragments to degradative effector functions have also been developed. These methods hold promise for inducing the degradation of proteins as both research tools and as a novel therapeutic approach. Protein aggregates are a pathophysiological feature of neurodegenerative diseases and are considered to have a causal role in pathology. Immunotherapy is emerging as a promising route towards their selective targeting, and a role of antibodies in the cytosol has been demonstrated in cell-based assays. This review will explore the mechanisms by which therapeutic antibodies engage and eliminate intracellularly aggregated proteins. We will discuss how future developments in intracellular antibody technology may enhance the therapeutic potential of such antibody-derived therapies.

Transmembrane protein 168 mutation reduces cardiomyocyte cell surface expression of Nav1.5 through αB-crystallin intracellular dynamics

Transmembrane protein 168 (TMEM168) was found to be localized on the nuclear membrane. A heterozygous mutation (c.1616G>A, p. R539Q) in TMEM168 was identified in patients with Brugada syndrome. This mutation reduced expression of cardiomyocyte sodium channel Nav1.5 via Nedd4-2 E3 ubiquitin ligase-induced ubiquitination and degradation. However, the detailed molecular mechanism provoked by the TMEM168 mutant remains unclear. Here, we demonstrated that small heat shock protein αB-crystallin, which can bind to Nav1.5 and Nedd4-2 and interfere with the association of both proteins, was strongly recruited from the cell surface to the perinuclear region because of the much higher interaction of αB-crystallin with the TMEM168 mutant than with wild-type TMEM168. Following knockdown of αB-crystallin in HL-1 cardiomyocytes, the interaction of Nav1.5 with Nedd4-2 was increased, despite a reduction of the expression level of Nav1.5. Moreover, αB-crystallin-mediated reduction of Nav1.5 expression was rescued in the presence of a proteasome inhibitor MG-132, suggesting the importance of the αB-crystallin-modulated ubiquitin-proteasome system for the stability of Nav1.5 expression. Collectively, the balance of molecular interactions among Nav1.5, Nedd4-2, and αB-crystallin plays a role in the regulation of cardiomyocyte cell surface expression of Nav1.5, and the TMEM168 mutant disturbs this balance, resulting in a decrease in Nav1.5 expression.

Potent antitumor effects of cell-penetrating peptides targeting STAT3 axis

To date, there are no inhibitors that directly and specifically target activated STAT3 and c-Myc in the clinic. Although peptide-based inhibitors can selectively block activated targets, their clinical usage is limited because of low cell penetration and/or serum stability. Here, we generated cell-penetrating acetylated (acet.) STAT3, c-Myc, and Gp130 targeting peptides by attaching phosphorothioated (PS) polymer backbone to peptides. The cell-penetrating peptides efficiently penetrated cells and inhibited activation of the intended targets and their downstream genes. Locally or systemically treating tumor-bearing mice with PS-acet.-STAT3 peptide at low concentrations effectively blocked STAT3 in vivo, resulting in significant antitumor effects in 2 human xenograft models. Moreover, PS-acet.-STAT3 peptide penetrated and activated splenic CD8+ T cells in vitro. Treating immune-competent mice bearing mouse melanoma with PS-acet.-STAT3 peptide inhibited STAT3 in tumor-infiltrating T cells, downregulating tumor-infiltrating CD4+ T regulatory cells while activating CD8+ T effector cells. Similarly, systemic injections of the cell-penetrating c-Myc and Gp130 peptides prevented pancreatic tumor growth and induced antitumor immune responses. Taken together, we have developed therapeutic peptides that effectively and specifically block challenging cancer targets, resulting in antitumor effects through both direct tumor cell killing and indirectly through antitumor immune responses.

Synergistic Interplay of Covalent and Non-Covalent Interactions in Reactive Polymer Nanoassembly Facilitates Intracellular Delivery of Antibodies

The primary impediments in developing large antibodies as drugs against intracellular targets involve their low transfection efficiency and suitable reversible encapsulation strategies for intracellular delivery with retention of biological activity. To address this, we outline an electrostatics-enhanced covalent self-assembly strategy to generate polymer-protein/antibody nanoassemblies. Through structure-activity studies, we down-select the best performing self-immolative pentafluorophenyl containing activated carbonate polymer for bioconjugation. With the help of an electrostatics-aided covalent self-assembly approach, we demonstrate efficient encapsulation of medium to large proteins (HRP, 44 kDa and β-gal, 465 kDa) and antibodies (ca. 150 kDa). The designed polymeric nanoassemblies are shown to successfully traffic functional antibodies (anti-NPC and anti-pAkt) to cytosol to elicit their bioactivity towards binding intracellular protein epitopes and inducing apoptosis.

Protein liposomes-mediated targeted acetylcholinesterase gene delivery for effective liver cancer therapy

BACKGROUND

Effective methods to deliver therapeutic genes to solid tumors and improve their bioavailability are the main challenges of current medical research on gene therapy. The development of efficient non-viral gene vector with tumor-targeting has very important application value in the field of cancer therapy. Proteolipid integrated with tumor-targeting potential of functional protein and excellent gene delivery performance has shown potential for targeted gene therapy.

RESULTS

Herein, we prepared transferrin-modified liposomes (Tf-PL) for the targeted delivery of acetylcholinesterase (AChE) therapeutic gene to liver cancer. We found that the derived Tf-PL/AChE liposomes exhibited much higher transfection efficiency than the commercial product Lipo 2000 and shown premium targeting efficacy to liver cancer SMMC-7721 cells in vitro. In vivo, the Tf-PL/AChE could effectively target liver cancer, and significantly inhibit the growth of liver cancer xenografts grafted in nude mice by subcutaneous administration.

CONCLUSIONS

This study proposed a transferrin-modified proteolipid-mediated gene delivery strategy for targeted liver cancer treatment, which has a promising potential for precise personalized cancer therapy.

Harnessing SLE Autoantibodies for Intracellular Delivery of Biologic Therapeutics

Intracellular delivery of therapeutic antibodies is highly desirable but remains a challenge for biomedical research and the pharmaceutical industry. Approximately two-thirds of disease-associated targets are found inside the cell. Difficulty blocking these targets with available drugs creates a need for technology to deliver highly specific therapeutic antibodies intracellularly. Historically, antibodies have not been believed to traverse the cell membrane and neutralize intracellular targets. Emerging evidence has revealed that anti-DNA autoantibodies found in systemic lupus erythematosus (SLE) patients can penetrate inside the cell. Harnessing this technology has the potential to accelerate the development of drugs against intracellular targets. Here, we dissect the mechanisms of the intracellular localization of SLE antibodies and discuss how to apply these insights to engineer successful cell-penetrating antibody drugs.

Overcoming Physiological Barriers to Nanoparticle Delivery-Are We There Yet?

The exploitation of nanosized materials for the delivery of therapeutic agents is already a clinical reality and still holds unrealized potential for the treatment of a variety of diseases. This review discusses physiological barriers a nanocarrier must overcome in order to reach its target, with an emphasis on cancer nanomedicine. Stages of delivery include residence in the blood stream, passive accumulation by virtue of the enhanced permeability and retention effect, diffusion within the tumor lesion, cellular uptake, and arrival at the site of action. We also briefly outline strategies for engineering nanoparticles to more efficiently overcome these challenges: Increasing circulation half-life by shielding with hydrophilic polymers, such as PEG, the limitations of PEG and potential alternatives, targeting and controlled activation approaches. Future developments in these areas will allow us to harness the full potential of nanomedicine.