In 1982, the Nobel Prize for Physiology or Medicine was shared between Bengt I. Samuelsson, Sune K. Bergström, and John R. Vane for the discovery of the role of prostaglandins and related biologically active substances in the body. Prostanoids like thromboxane and prostaglandins like prostacyclin are synthesized by the enzyme cyclooxygenase (COX) [EC 1.14.99.1], a prostaglandin-endoperoxide synthase (PTGS). Prostaglandin is produced in areas of tissue damage or infection and is causing inflammation and pain. Prostacyclin removes blood clots and dilates blood vessels to increase blood flow. Thromboxane triggers the clotting of platelets and constricts blood vessels to decrease blood loss.

COX, therefore, is a common target for pain relief and anti-inflammatory drugs. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as the blockbusters Aspirin®, Paracetamol and Ibuprofen, are well-known COX inhibitors. They are used for the treatment of moderate pains like headache, toothache, or migraine. For acute pain as caused by severe injuries or trauma, opioids might be used.

Aspirin® (acetylsalicylic acid) (1) is used to reduce pain, fever, and/or inflammation, and as an antithrombotic. It carries a free carboxylic acid function, which can be used for linker attachment.

Paracetamol (para-hydroxyacetanilide) (2) is a non-opioid analgesic agent used to treat fever and mild to moderate pain. It is commonly used just as Aspirin® and bears a phenol group to conjugate a linker.

Ibuprofen (3) is a nonsteroidal anti-inflammatory drug (NSAID) that is used to relieve pain, fever, and inflammation. This includes painful menstrual periods, migraines, and rheumatoid arthritis. Similar to Aspirin® it carries a free carboxylic acid and no other functional group - an ideal constellation for linker attachment.

Morphine (4) and Codeine (5) are strong opiates found in natural opium. Primarily, they are used as analgesic in severe cases of both acute (myocardial infarction, kidney stones, etc.) and chronic pain. Both act directly on the central nervous system (CNS) and induce analgesia and alter perception and emotional response to pain. Physical and psychological dependence and tolerance may develop upon repeated administration. Consequently, this class of compounds is subject to the laws of The Controlled Drugs and Substances Act (CDSA). Regarding their chemical structures, Morphine has a phenol function, which can be used for linker attachment, whereas this hydroxy group is blocked by methylation in the case of Codeine. In this case, the tertiary amino function at the opposite side of the molecule can be alkylated by a p-aminobenzyl linker (PAB).

Chemical structures of common drugs used for pain treatment: Aspirin® (1), Paracetamol (2), Ibuprofen (3), Morphine (4), Codeine (5). Possible points of linker attachment are highlighted in red.

As briefly mentioned above, the phenol function of Morphine can be used for further conjugation. In our example, a methyl(2-methylamino)ethyl)carbamate linker is used. This moiety is attached to a dithioethylcarbamate, which releases 1,3-oxathiolane-2-one after reductive cleavage of the disulfide bond. In the case of Codeine, the tertiary amino group can be alkylated by a PAB linker, which is further conjugated to a peptidic valyl-alanine linker that fragmentizes upon proteolytic cleavage by Cathepsin B. In both cases, the linker allows to attach the drug molecule to any type of carrier and to release it in a traceless manner upon induced cleavage.

Conjugation of Morphine and Codeine, respectively, to a specific linker and traceless drug release upon trigger-induced self-immolation.

For the development of constructs for combination therapy, we envisioned the attachment of different drug-linker conjugates onto the same carrier molecule, e.g. protein or antibody, carbon nanoparticles, polymers, metals or metal oxides. Therefore, different scenarios need to be distinguished.

1. If the carrier bears only one functional group, a consecutive approach is employed. First, around 50% of the surface functional groups will be covered with drug-linker conjugate 1 (= cargo 1). Afterwards, drug-linker conjugate 2 (= cargo 2) is attached. In this case, only one type of functional group needs to be installed on the carrier. However, batch-to-batch consistency and cargo ratios on the surface are difficult to control.
   
   
   
2. If the carrier is equipped with orthogonal functional groups, cargos with complementary reactive groups can be designed. This strategy allows fine-tuning of the cargo ratios on the surface and better batch-to-batch reproducibility. However, the overall design of surface and cargo functional groups is more challenging.
   
   

   Consecutive attachment of two different drug-linker constructs with orthogonal functional groups on a carrier bearing two types of complementary functional groups.

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Modulating Therapeutic Activity and Toxicity of Pyrrolobenzodiazepine Antibody-Drug Conjugates with Self-Immolative Disulfide Linkers; T. H. Pillow, M. Schutten, S. F. Yu, R. Ohri, J. Sadowsky, K. A. Poon, W. Solis, F. Zhong, G. Del Rosario, M. A. T. Go, J. Lau, S. Yee, J. He, L. Liu, C. Ng, K. Xu, D. D. Leipold, A. V. Kamath, D. Zhang, L. Masterson, S. J. Gregson, P. W. Howard, F. Fang, J. Chen, J. Gunzner-Toste, K. K. Kozak, S. Spencer, P. Polakis, A. G. Polson, J. A. Flygare, J. R. Junutula; Mol. Cancer. Ther. 2017; 16: 871-878. https://doi.org/10.1158/1535-7163.MCT-16-0641

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ABC transporters as multidrug resistance mechanisms and the development of chemosensitizers for their reversal; C. H. Choi; Cancer Cell Int 2005; 5: 30. https://doi.org/10.1186/1475-2867-5-30

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