Compartmentalization is one of the main characteristics that define living systems. Creating a physically separated microenvironment allows nature a better control over biological processes, as is clearly specified by the role of organelles in living cells. Inspired by this phenomenon, researchers have developed a range of different approaches to create artificial organelles: compartments with catalytic activity that add new function to living cells. In this review we will discuss three complementary lines of investigation. First, orthogonal chemistry approaches are discussed, which are based on the incorporation of catalytically active transition metal‐containing nanoparticles in living cells. The second approach involves the use of premade hybrid nanoreactors, which show transient function when taken up by living cells. The third approach utilizes mostly genetic engineering methods to create bio‐based structures that can be ultimately integrated with the cell's genome to make them constitutively active. The current state of the art and the scope and limitations of the field will be highlighted with selected examples from the three approaches.

Activatable cell-penetrating peptides are of great interest in drug delivery because of their enhanced selectivity which can be controlled by the external stimuli that trigger their activation. The use of a specific enzymatic reaction to trigger uptake of an inert peptide offers a relevant targeting strategy because the activation process takes place in a short time and only in areas where the specific cell surface enzyme is present. To this aim, the lysine side chain of Tat peptides was modified with an enzyme-cleavable domain of minimal size. This yielded blocked Tat-peptides which were inactive but that could be activated by coincubation with the selected enzymes.

Photodynamic therapy (PDT) is a highly promising therapeutic modality for cancer treatment. The development of stimuli‐responsive photosensitizer nanomaterials overcomes certain limitations in clinical PDT. Herein, we report the rational design of a highly sensitive PEGylated photosensitizer‐peptide nanofiber (termed PHHPEG6 NF) that selectively aggregates in the acidic tumor and lysosomal microenvironment. These nanofibers exhibit acid‐induced enhanced singlet oxygen generation, cellular uptake, and PDT efficacy in vitro, as well as fast tumor accumulation, long‐term tumor imaging capacity and effective PDT in vivo. Moreover, based on the prolonged presence of the fluorescent signal at the tumor site, we demonstrate that PHHPEG6 NFs can also be applied for prognostic monitoring of the efficacy of PDT in vivo, which would potentially guide cancer treatment. Therefore, these multifunctional PHHPEG6 NFs allow control over the entire PDT process, from visualization of photosensitizer accumulation, via actual PDT to the assessment of the efficacy of the treatment.

Polymersome nanoreactors that can be employed as artificial organelles have gained much interest over the past decades. Such systems often include biological catalysts (i.e., enzymes) so that they can undertake chemical reactions in cellulo. Examples of nanoreactor artificial organelles that acquire metal catalysts in their structure are limited, and their application in living cells remains fairly restricted. In part, this shortfall is due to difficulties associated with constructing systems that maintain their stability in vitro, let alone the toxicity they impose on cells. This study demonstrates a biodegradable and biocompatible polymersome nanoreactor platform, which can be applied as an artificial organelle in living cells. The ability of the artificial organelles to covalently and non‐covalently incorporate tris(triazolylmethyl)amine‐Cu(I) complexes in their membrane is shown. Such artificial organelles are capable of effectively catalyzing a copper‐catalyzed azide‐alkyne cycloaddition intracellularly, without compromising the cells’ integrity. The platform represents a step forward in the application of polymersome‐based nanoreactors as artificial organelles.

Photodynamic therapy (PDT) is a highly promising therapeutic modality for cancer treatment. The development of stimuli-responsive photosensitizer nanomaterials overcomes certain limitations in clinical PDT. Herein, we report the rational design of a highly sensitive PEGylated photosensitizer-peptide nanofiber (termed PHHPEG 6 NF) that selectively aggregates in the acidic tumor and lysosomal microenvironment. These nanofibers exhibit acid-induced enhanced singlet oxygen generation, cellular uptake, and PDT efficacy in vitro, as well as fast tumor accumulation, long-term tumor imaging capacity and effective PDT in vivo. Moreover, based on the prolonged presence of the fluorescent signal at the tumor site, we demonstrate that PHHPEG 6 NFs can also be applied for prognostic monitoring of the efficacy of PDT in vivo, which would potentially guide cancer treatment. Therefore, these multifunctional PHHPEG 6 NFs allow control over the entire PDT process, from visualization of photosensitizer accumulation, via actual PDT to the assessment of the efficacy of the treatment.