Self-immolative Spacers Research Articles

Amine-Carbamate Self-Immolative Spacers Counterintuitively Release 3° Alcohol at Much Faster Rates than 1° Alcohol Payloads.

Self-immolative (SI) spacers are degradable chemical connectors widely used in prodrugs and drug conjugates to release pharmaceutical ingredients in response to specific stimuli. Amine-carbamate SI spacers are particularly versatile, as they have been used to release different hydroxy cargos, ranging from 2° and 3° alcohols to phenols and oximes. In this work, we describe the ability of three amine-carbamate SI spacers to release three structurally similar imidazoquinoline payloads, bearing either a 1°, a 2° or a 3° alcohol as the leaving group. While the spacers showed comparable efficacy at releasing the 2° and 3° alcohols, the liberation of the 1° alcohol was much slower, unveiling a counterintuitive trend in nucleophilic acyl substitutions. The release of the 1° alcohol payload was only possible using a SI spacer bearing a pyrrolidine ring and a tertiary amine handle, which opens the way to future applications in drug delivery systems.

Advanced Pyrrolidine-Carbamate Self-Immolative Spacer with Tertiary Amine Handle Induces Superfast Cyclative Drug Release.

Amine‐carbamate self‐immolative (SI) spacers represent practical and versatile tools in targeted prodrugs, but their slow degradation mechanism limits drug activation at the site of disease. We engineered a pyrrolidine‐carbamate SI spacer with a tertiary amine handle which strongly accelerates the spacer cyclization to give a bicyclic urea and the free hydroxy groups of either cytotoxic (Camptothecin) or immunostimulatory (Resiquimod) drugs. In silico conformational analysis and pK a calculations suggest a plausible mechanism for the superior efficacy of the advanced SI spacer compared to state‐of‐art analogues.

The architectural evolution of self-immolative polymers

Self-immolative polymers (SIPs) depolymerize to small molecules through a cascade of reactions following cleavage of the polymer backbone or a specific terminal or focal point moiety by a stimulus. They have been developed using the principles of self-immolative spacers and low ceiling temperature (Tc) polymers. Key developments over the past couple of decades have been the polymerization of spacers to enable long reaction cascades, and the introduction of stimuli-responsive end-caps which have allowed depolymerization to be triggered in a controlled manner in response to a wide array of stimuli. This review will focus on the architectural evolution of SIPs over the past two decades from oligomers to dendrimers, linear polymers, cyclic polymers, graft copolymers, networks, and hyperbranched systems. We will discuss how the architecture influences the triggering and propagation of the reaction cascade and highlight how different architectures can provide advantages and disadvantages in terms of their synthesis and properties.

Fast Cyclization of a Proline-Derived Self-Immolative Spacer Improves the Efficacy of Carbamate Prodrugs.

Self-immolative (SI) spacers are sophisticated chemical constructs designed for molecular delivery or material degradation. We describe herein a (S)-2-(aminomethyl)pyrrolidine SI spacer that is able to release different types of anticancer drugs (possessing either a phenolic or secondary and tertiary hydroxyl groups) through a fast cyclization mechanism involving carbamate cleavage. The high efficiency of drug release obtained with this spacer was found to be beneficial for the in vitro cytotoxic activity of protease-sensitive prodrugs, compared with a commonly used spacer of the same class. These findings expand the repertoire of degradation machineries and are instrumental for the future development of highly efficient delivery platforms.

Synthesis and Biological Evaluation of RGD⁻Cryptophycin Conjugates for Targeted Drug Delivery.

Cryptophycins are potent tubulin polymerization inhibitors with picomolar antiproliferative potency in vitro and activity against multidrug-resistant (MDR) cancer cells. Because of neurotoxic side effects and limited efficacy in vivo, cryptophycin-52 failed as a clinical candidate in cancer treatment. However, this class of compounds has emerged as attractive payloads for tumor-targeting applications. In this study, cryptophycin was conjugated to the cyclopeptide c(RGDfK), targeting integrin αvβ3, across the protease-cleavable Val-Cit linker and two different self-immolative spacers. Plasma metabolic stability studies in vitro showed that our selected payload displays an improved stability compared to the parent compound, while the stability of the conjugates is strongly influenced by the self-immolative moiety. Cathepsin B cleavage assays revealed that modifications in the linker lead to different drug release profiles. Antiproliferative effects of Arg-Gly-Asp (RGD)–cryptophycin conjugates were evaluated on M21 and M21-L human melanoma cell lines. The low nanomolar in vitro activity of the novel conjugates was associated with inferior selectivity for cell lines with different integrin αvβ3 expression levels. To elucidate the drug delivery process, cryptophycin was replaced by an infrared dye and the obtained conjugates were studied by confocal microscopy.

Mechanistic Evaluation of Bioorthogonal Decaging with trans-Cyclooctene: The Effect of Fluorine Substituents on Aryl Azide Reactivity and Decaging from the 1,2,3-Triazoline.

Bioorthogonal prodrug activation/decaging strategies need to be selective, rapid and release the drug from the masking group upon activation. The rates of the 1,3-dipolar cycloaddition between a trans-cyclooctene (TCO) and a series of fluorine-substituted azido-PABC self-immolative spacers caging two model drugs, and subsequent release from the 1,2,3-triazoline are reported. As the number of fluorine substituents on the PABC linker increases from one to four, the rate of cycloaddition increases by almost one order of magnitude. Using a combination of fluorescence, 1H/19F NMR, and computational experiments, we have been able to determine how substituents on the PABC ring can influence the degradation rates and also the product distribution of the 1,2,3-triazoline. We have also been able to determine how these substituents influence the rate of imine hydrolysis and 1,6-self-immolation decaging rates of the generated anilines. The NMR and computational studies demonstrate that fluorine substituents on the aromatic ring lower the transition state energy required for converting the triazoline to the imine or aziridine intermediates via extrusion of diatomic nitrogen, and that in the case of a tetrafluoro substituted aromatic ring, it is the imine hydrolysis and 1,6-self-immolation that is rate-limiting. This knowledge further enhances the understanding of factors which influence the stability of triazolines, and enables potential applications of fluorinated aromatics, in particular, perfluorinated aromatics, in synthetic chemistry and sustained-release drug delivery systems.

Syntheses and kinetic studies of cyclisation-based self-immolative spacers.

Kinetic analysis of the disassembly of self-immolative spacers based on cyclisation processes was performed. Five compounds were synthesized belonging to two different series, and their kinetic constants were determined. Electron-donating substituents gave a slight acceleration but the main effect was steric, and the Thorpe-Ingold effect was indeed particularly effective. Comparison with the self-immolative spacers based on elimination processes showed that cyclisations gave comparable or lower rate, but the corresponding spacers are more difficult to modulate.

Glyceride-Mimetic Prodrugs Incorporating Self-Immolative Spacers Promote Lymphatic Transport, Avoid First-Pass Metabolism, and Enhance Oral Bioavailability.

First-pass hepatic metabolism can significantly limit oral drug bioavailability. Drug transport from the intestine through the lymphatic system, rather than the portal vein, circumvents first-pass metabolism. However, the majority of drugs do not have the requisite physicochemical properties to facilitate lymphatic access. Herein, we describe a prodrug strategy that promotes selective transport through the intestinal lymph vessels and subsequent release of drug in the systemic circulation, thereby enhancing oral bioavailability. Using testosterone (TST) as a model high first-pass drug, glyceride-mimetic prodrugs incorporating self-immolative (SI) spacers, resulted in remarkable increases (up to 90-fold) in TST plasma exposure when compared to the current commercial product testosterone undecanoate (TU). This approach opens new opportunities for the effective development of drugs where oral delivery is limited by first-pass metabolism and provides a new avenue to enhance drug targeting to intestinal lymphoid tissue.

ChemInform Abstract: Self‐Immolative Spacers: Kinetic Aspects, Structure—Property Relationships, and Applications

AbstractReview: 150 refs.

Self-immolative spacers: kinetic aspects, structure-property relationships, and applications.

Self-immolative spacers are covalent assemblies tailored to correlate the cleavage of two chemical bonds after activation of a protective part in a precursor: Upon stimulation, the protective moiety is removed, which generates a cascade of disassembling reactions leading to the temporally sequential release of smaller molecules. Originally introduced to overcome limitations for drug delivery, self-immolative spacers have gained wide interest in medicinal chemistry, analytical chemistry, and material science. For most applications, the kinetics of the disassembly of the activated self-immolative spacer governs functional properties. This Review addresses kinetic aspects of self-immolation. It provides information for selecting a particular self-immolative motif for a specific demand. Moreover, it should help researchers design kinetic experiments and fully exploit the rich perspectives of self-immolative spacers.

Triggered degradation of poly(ester amide)s via cyclization of pendant functional groups of amino acid monomers

Poly(ester amide)s (PEAs) are of interest for a diverse range of applications as their structures and properties can be readily tuned through the incorporation of a wide variety of monomers. In this work, the incorporation of the amino acids L-2,4-diaminobutyric acid (DAB) and homocysteine (HCY) was investigated with the aim of imparting stimuli-responsive degradation properties to PEAs. First, small molecule model compounds were prepared and studied to demonstrate that upon revealing the pendant γ-amine or γ-thiol of DAB and HCY esters, intramolecular cyclizations to 5-membered lactams and thiolactones respectively were much more rapid than background ester hydrolysis. Subsequently, monomers containing these DAB and HCY self-immolative spacers were prepared and incorporated into PEAs such that cleavage of protecting groups on the pendant moieties by stimuli including acid and the reducing agent DTT induced cyclization reactions that directly cleaved esters in the PEA backbone. The degradation of these polymers both in solution and in films was studied, demonstrating stimuli-triggered degradation was more rapid than background polymer degradation. In addition, to demonstrate that the approach could be readily extended to various stimuli, a photochemically responsive PEA was prepared by simply changing the protecting group on the pendant amine. This intramolecular cyclization strategy involving pendant functional groups should therefore be useful for the development of a wide range of PEAs and other polymers for which it is desirable to initiate degradation under specified conditions.

A Self-Immolative Spacer That Enables Tunable Controlled Release of Phenols under Neutral Conditions

A current challenge in the area of responsive materials is the design of reagents and polymers that provide controlled release of phenols in environments that are less polar than water. In these contexts, a molecular strategy that enables release of nearly any phenol with predictable and tunable rates and without complication from background hydrolysis would substantially increase the precision with which materials can be designed to respond to a particular signal. This Article addresses this problem at the fundamental level by describing the design, synthesis, and physical-organic characterization of two small molecule self-immolative spacers that are capable of releasing phenols in organic and mixed organic-aqueous solutions. The rate of release from these small molecule model systems is predictable and tunable, such that nearly any type of phenol, regardless of pK(a) value, can be released in neutral solutions without complications from nonspecific background release due to hydrolysis. Furthermore, the release properties of the spacers can be predicted from bond length and conformation data (obtained from crystal structures). On the basis of these results, it should now be possible to incorporate these design elements into materials to enable precise response properties in environments that are not 100% aqueous.

Design, synthesis, and cyclization of 4-aminobutyric acid derivatives: potential candidates as self-immolative spacers

Self-immolative spacers have gained significant interest in recent years due to their utility in numerous prodrug, sensor and drug delivery systems. However, there are a very limited number of spacers that are capable of undergoing spontaneous and rapid reactions under mild conditions. To address this need, 4-aminobutyric acid derivatives were explored as a potential class of self-immolative spacers. Using a modular approach, eleven N- and α-substituted derivatives of 4-aminobutyric acid were synthesized, and their intramolecular cyclizations to γ-lactams were studied. Kinetics experiments were carried out at physiological pH and temperature, and the observed half-lives for the spacers ranged from 2 to 39 s, depending on the molecular structure. In addition, the pH dependence of the cyclization rate was also explored and it was found that cyclization still occurred rapidly at mildly acidic pH. Therefore, this class of compounds exhibits promise for incorporation into a variety of self-immolative systems where rapid cyclization reactions are desired.

Supramolecular hydrogels for enzymatically triggered self-immolative drug delivery

For the first time the combination of self-immolative spacers and supramolecular hydrogels has been tested in enzyme triggered drug release. Low-molecular weight drug-gelator conjugates have been prepared, which contain a gel forming lysine moiety linked to model drugs (benzylamine and phenethylamine) through a self-immolating spacer ( p-aminobenzyloxycarbonyl). In the presence of trypsin the amide linkage between the gelator moiety and the spacer is hydrolyzed leading to the release of the model drug. This approach provides with distinct advantages, such as sustained release or versatility associated to the use of supramolecular hydrogels and self-immolative spacers, respectively.

Design, Synthesis, and Biological Evaluation of Antibody−Drug Conjugates Comprised of Potent Camptothecin Analogues

Antibody-drug conjugates (ADCs) were prepared with potent camptothecin analogues attached to monoclonal antibodies (mAbs) via dipeptide or glucuronide-based linkers. Aniline-containing camptothecin analogues were employed to provide a site of linker attachment via carbamate bonds that would be stable in circulation. The camptothecin analogues, 7-butyl-10-amino-camptothecin and 7-butyl-9-amino-10,11-methylenedioxy-camptothecin, are generally 10-1000 times more potent than camptothecin. Dipeptide and glucuronide drug linkers were employed containing self-immolative spacers that release drug following lysosomal degradation upon ADC internalization into antigen-positive cell lines. The camptothecin drug linkers were conjugated to three antibodies: chimeric BR96, chimeric AC10, and humanized 1F6, which bind to the Lewis-Y antigen on carcinomas, CD30 on hematologic malignancies, and CD70 present on hematologic malignancies and renal cell carcinoma, respectively. ADCs bearing the potent camptothecin analogue, 7-butyl-9-amino-10,11-methylenedioxy-camptothecin, were highly potent and immunologically specific on a panel of cancer cell lines in vitro, and efficacious at well-tolerated doses in a renal cell carcinoma xenograft model.