Function of spermidine and its metabolizing enzymes in the three most important disabling human viruses.
More than 76 million people worldwide are infected with the human immunodeficiency virus (HIV) which causes acquired immune deficiency syndrome (AIDS). There are now 37 million people living with the infection 
due to a highly effective antiretroviral therapy (HAART). Human immunodeficiency is caused by a collection of viruses, with human immunodeficiency virus 1 (HIV-1) being the most prevalent and HIV-2 being less pathogenic. HIV-1 is an RNA virus of the genus Lentivirus which belongs to the family of Retroviridae
. The disease leads to progressive loss of CDC4+
cells, causing infectious and immune abnormalities and oncological complications. Meanwhile, antiretroviral drugs are highly efficient for patients who can access and adhere to them, thus leading to durable and probable long-life suppression of viral replication. HIV is a retrovirus that integrates its DNA into the human genome. The main receptor for HIV virus is the CD4 receptor together with two chemokine receptors, CCR5 and CXCR4. These receptors are expressed on T lymphocytes, macrophages, and monocytes. After fusion and uncoating, the RNA is reverse transcribed into DNA. Subsequently, HIV DNA is transcribed to viral mRNA, after import in the nucleus. These HIV mRNAs are then exported to the cytoplasm where translation occurs to make viral proteins and eventually mature virions. Each step of the HIV life cycle, i.e., HIV entry, reverse transcription, integration, and protein maturation, can be targeted by antiretroviral drugs. EIF5A plays an important role in HIV-1 replication since it is a “cellular cofactor” of the HIV-1 trans-activator protein Rev (Figure 3
3. Spermidine as a Hallmark in the Apicomplexan Parasite P. falciparum
Apicomplexan parasites represent a phylum with an apical complex; a conical structured organelle at the apical end of the cell that enables the parasite to penetrate the host by micronemes, rhoptries, conoid, and polar rings 
. The apicomplexan parasites of medical importance include Plasmodium
spp. (causing malaria), Cryptosporidium parvum
(causing Cryptosporidiosis), Babesia
sp. (causing Babesiosis), and Toxoplasma gondii
(causing Toxoplasmosis). Most of the apicomplexan parasites are sporozoans but some of them also produce oocytes.
Almost 50 years after the establishment of the WHO Global Malaria Programme, malaria remains a major global health problem with an estimated 627,000 deaths in 2020, according to a WHO report published in 2021 
. Although the number of deaths decreased between 2000 and 2019, SARS-CoV-2 refueled the rate of infection. Moreover, the death rate of children under the age of five remained at 72%. The development of the RTS,S vaccine against the parasite’s circumsporozoite protein was a milestone although it showed only a short-lived immune protection in phase III clinical trials in different African countries 
. Therefore, next-generation vaccines with prolonged immune protection are necessary and are beyond the horizon 
. Eradication of malaria is a major problem, since its causative agent, P. falciparum
, has a complex life cycle and can remain latent within the human host 
. Conclusively, eradication of malaria will only be successful with a combined therapy of vaccination and small molecules due to the developing resistance against registered drugs. Plasmodium
ssp. perform de novo polyamine biosynthesis but also use a salvage pathway for the uptake of Put and Spd 
in the infected erythrocyte. The first reports about highly specific Put and Spd carriers demonstrated novel transporters with different kinetic parameters compared to transporters in the uninfected erythrocyte 
. Later, radioisotope flux techniques proved the uptake of Put and Spd in a temperature dependent process from the infected erythrocyte 
is the only apicomplexan parasite which contains a full set of key enzymes involved in the biosynthesis of spermidine (the core pathway), i.e., ODC, AdoMetDC, and SpdS (Figure 4
), and a conserved pathway leading to the posttranslational modification of hypusine in eIF5A.
Figure 4. De novo
biosynthesis of polyamines (PAs) in Plasmodium falciparum
has a core PA pathway with ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (AdoMetDC), and spermidine synthase (SpdS). A bifunctional AdoMetDC and a SpS producing Spm are peculiar for this pathway in Plasmodium
. The parasite is also able to use Put and Spd from the salvage pathway of the infected host red blood cell (RBC). Hypusine biosynthesis is highly conserved. Deoxhypusine synthase (DHS) catalyzes the transfer of the aminopropyl moiety to lysine 50 in the precursor protein and deoxyhypusine hydroxylase (DOHH) introduces the hydroxyl group to carbon 9 in the side chain.
4. Spermidine Biosynthesis and Its Metabolizing Pathways in the Trypanosomatids Are Unique
The trypanosomatids cause a variety of insect-born pathogenic, infectious, human diseases such as human African trypanosomiasis (T. brucei
), Chagas disease (T. cruzi)
, and Leishmaniasis (Leishmania
species). According to WHO reports 
, an estimated 18 million people are infected worldwide. In trypanosomatids, a core PA pathway and a trypanothione pathway exist as unique metabolic pathways 
. They both utilize spermidine as a key metabolite (Figure 5
). Most of the enzymes of both pathways are essential for survival, growth, and infectivity. One peculiar feature of the core PA pathway is the occurrence of pseudoenzymes that evolved from enzymes into regulators 
. In this context, AdoMetDC and DHS are outstanding examples (Figure 5
). However, trypanosomatids lack catabolizing enzymes such as SMOX, SSAT, and APAO although some catabolites produced by these enzymes appear in the parasite.
Figure 5. Schematic representation of a core PA pathway (A), and a Trypanothione pathway (B) in Trypanosomatids. In both pathways spermidine represents an important key metabolite. Pseudoenzymes, i.e., DHS and AdoMetDC of a core PA pathway are marked in green. The right part of the figure: Cys (cysteine) and Glu (glutamate) are covalently linked by γECS (gamma-glutamylcysteine synthetase) to form γEC (gamma-glutamylcysteine). Gly (Glycine) is bound to γEC by glutathione synthetase (GS) producing glutathione (GSH). Finally, trypanothione synthetase (TryS) synthesizes T(SH)2 by binding two GSH molecules to a Spd molecule. Trypanothione reductase (TyrR) catalyzes oxidized T(SH)2 reduction to TS2.
5. The Impact to Develop Inhibitors against Enzymes Involved in Spermidine Biosynthesis and Metabolism Treating Human African Trypanomiasis (HAT)
Currently, the only polyamine biosynthetic inhibitor that has been registered for HAT treatment is the irreversible ODC inhibitor DFMO 
. It is mostly administered in combination with other drugs (nifurtimox, suramin, melarsoprol) depending on the type of Trypanosomes (T. gambiense
or T. rhodesiense
) and the disease state of infection (presence or absence of central nervous symptoms). Generally, DFMO is used intravenously for late-stage T. gambiense
infection. However, the drug regimen is complicated and hampers patients’ compliance due to the high dosing course of 200 mg/kg for 7 days. DFMO also targets ODC from humans. However, selective toxicity to the enzyme from the parasite is achieved due to differences in intracellular turnover rates 
. Hence, pentamidine is still recommended for the first stage of T. gambiense
infection. Drug development for polyamine biosynthetic inhibitors in Trypanosomes is hampered by the fact that apart from the core polyamine pathway, a salvage pathway exists.