Covid-19 treatment – why chloroquine?

28th March 2020, Dr Chee L Khoo

Covid-19 belong to the subfamily Coronavirinae in the family of Coronaviridae of the order Nidovirales, and this subfamily includes four genera: α-coronavirus, β-coronavirus, γ-coronavirus, and δ-coronavirus. The new 2019 nCoV (or Covid-19), which belongs to β-coronavirus can infect the lower respiratory tract and cause pneumonia in humans, but it seems that the symptoms are milder than SARS and MERS. Cell entry of coronaviruses depends on binding of the viral spike (S) proteins to the cellular receptors, angiotensin converting enzyme 2 (ACE2) and on S protein priming by host cell proteases.

We are familiar with angiotensin converting enzyme (ACE) which converts angiotensin I to angiotensin II. Of course, angiotensin II, amongst many of its actions cause vasoconstriction, increase vasopressin and aldosterone secretion. What about ACE2? ACE2 is an enzyme attached to the outer surface membrane of cells in the lungs, arteries, heart, kidney, and intestines. ACE2 cleaves angiotensin II into angiotensin 1 thereby reducing vasoconstriction.

Unfortunately, ACE2 also serves as the entry point into cells for some coronaviruses including SARS-CoV, the virus that causes SARS and SARS-CoV-2, the virus that causes COVID-19. This might lead some to believe that decreasing the levels of ACE2, in cells, might help in fighting the infection. according to studies conducted on mice, the interaction of the spike protein of the coronavirus with ACE2 induces a drop in the levels of ACE2 in cells through internalisation and degradation of the protein and hence may contribute to lung damage (3,4).

Just in case, you go off stopping your patients’ ACE inhibitors or ARB, ACE2 has been shown to have a protective effect against virus-induced lung injury by increasing the production of angiotensin I (3,4). The jury is still out.

What does chloroquine do?

Quinine is a compound found in the bark of Cinchona trees native to Peru and was the previous drug of choice against malaria. Chloroquine is an amine acidotropic form of quinine that was synthesised in Germany by Bayer in 1934 and emerged approximately 70 years ago as an effective substitute for natural quinine [1,2]. Hydroxychloroquine differs from chloroquine by the presence of a hydroxyl group at the end of the side chain: N-ethyl substituent is β-hydroxylated.

Chloroquine is also utilised in the treatment of autoimmune diseases (including rheumatoid arthritis and systemic lupus erythematosus) in the mid-1990s, due to its tolerability, rare toxicity reports, inexpensive cost and immunomodulatory properties, chloroquine repurposing was explored against human immunodeficiency virus (HIV) and other viruses associated with inflammation and was found to be efficient in inhibiting their replication cycle.

The multiple molecular mechanisms by which chloroquine can achieve such results remain to be further explored. So far, studies reveal that the anti-viral activities of chloroquine may work via (5):

  • Increasing endosomal pH required for virus/cell fusion, as well as
  • Interfering with the glycosylation of cellular receptors of SARS-CoV
  • Interfere with the post-translational modification of viral proteins
  • Act on the immune system through cell signalling and regulation of pro-inflammatory cytokines

The Evidence

Thus, in theory, chloroquine has anti-viral properties. But does it work in practice? In vitro (i.e. in the petri dish), growth of many different viruses is inhibited in cell culture by both chloroquine and hydroxychloroquine, including the SARS coronavirus. Similar anti-viral activity has been seen in mice for a variety of viruses, including human coronavirus OC43, enterovirus EV-A71, Zika virus and influenza A H5N1.

What about patients in the real world?

Chloroquine did not prevent influenza infection in a randomised, double-blind, placebo-controlled clinical trial (6), and had no effect on dengue-infected patients in a randomised controlled trial in Vietnam (7).

In a non-human primate model of chikingunya infection, chloroquine treatment was shown to exacerbate acute fever and delay the cellular immune response, leading to an incomplete viral clearance (8). A clinical trial conducted during the chikungunya outbreak in 2006 in Réunion Island showed that oral chloroquine treatment did not improve the course of the acute disease (9). The only modest effect of chloroquine in the therapy of human virus infection was found for chronic hepatitis C: an increase of the early virological response to pegylated interferon plus ribavirin (10) and, in a small sample size pilot trial in non-responder HCV patients, a transient viral load reduction (11) were observed.

In a recent publication, Gao et al indicate that “results from more than 100 patients infected with Covid-19 demonstrated that chloroquine phosphate is superior to the control treatment in inhibiting the exacerbation of pneumonia, improving lung imaging findings, promoting a virus negative conversion, and shortening the disease course” (12). Unfortunately, no data were released to support their conclusions.

“Although the long use of this drug in malaria therapy demonstrates the safety of acute chloroquine administration to humans, one cannot ignore the minor risk of macular retinopathy, the risk of which depends on the cumulative dose.  Also there are some reports, although rare, that chloroquine can  cause cardiomyopathy.” 

There are at least 10 clinical trials currently underway looking into chloroquine as treatment against covid-19. There are also other clinical trials that are looking at inhibiting key components of the coronavirus infection lifecycle. These include viral entry into the host cell (blocked by umifenovir, chloroquine or interferon), viral replication (blocked by lopinavir/ritonavir, ASC09 or darunavir/cobicistat, which inhibit the 3C-like protease (3CLpro)) and viral RNA synthesis (inhibited by remdesivir, favipiravir, emtricitabine/tenofovir alafenamide or ribavirin).

Interesting times ahead.


  3. Imai, Y., Kuba, K., Rao, S., Huan, Y., Guo, F., Guan, B., Penninger, J. M. (2005). Angiotensin-converting enzyme 2 protects from severe acute lung failure. Nature, 436(7047), 112–116.
  4. Kuba, K., Imai, Y., Rao, S., Gao, H., Guo, F., Guan, B., Penninger, J. M. (2005). A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury. Nature Medicine, 11(8), 875–879.
  5. Christian A. Devaux , Jean-Marc Rolain , Philippe Colson , Didier Raoult , New insights on the antiviral effects of chloroquine against coronavirus: what to expect for COVID-19?, International Journal of Antimicrobial Agents (2020), doi:
  6. Paton, N.I., Lee, L., Xu, Y., Ooi, E.E., Cheung, Y.B., Archuleta, S., Wong, G., Wilder-Smith, A., Smith, A.W., 2011. Chloroquine for influenza prevention: a randomised, doubleblind, placebo controlled trial. Lancet Infect. Dis. 11, 677–683.
  7. Tricou, V., Minh, N.N., Van, T.P., Lee, S.J., Farrar, J., Wills, B., Tran, H.T., Simmons, C.P., 2010. A randomized controlled trial of chloroquine for the treatment of dengue in Vietnamese adults. PLoS Neglected Trop. Dis. 4, e785.
  8. Roques, P., Thiberville, S.-D., Dupuis-Maguiraga, L., Lum, F.-M., Labadie, K., Martinon, F., Gras, G., Lebon, P., Ng, L.F.P., de Lamballerie, X., Le Grand, R., 2018. Paradoxical effect of chloroquine treatment in enhancing chikungunya virus infection. Viruses 10.
  9. De Lamballerie, X., Boisson, V., Reynier, J.-C., Enault, S., Charrel, R.N., Flahault, A., Roques, P., Le Grand, R., 2008. On chikungunya acute infection and chloroquine treatment. Vector Borne Zoonotic 8, 837–839.
  10. Helal, G.K., Gad, M.A., Abd-Ellah, M.F., Eid, M.S., 2016. Hydroxychloroquine augments early virological response to pegylated interferon plus ribavirin in genotype-4 chronic hepatitis C patients. J. Med. Virol. 88, 2170–2178.
  11. Peymani, P., Yeganeh, B., Sabour, S., Geramizadeh, B., Fattahi, M.R., Keyvani, H., Azarpira, N., Coombs, K.M., Ghavami, S., Lankarani, K.B., 2016. New use of an old drug: chloroquine reduces viral and ALT levels in HCV non-responders (a randomized, triple-blind, placebo-controlled pilot trial). Can. J. Physiol. Pharmacol. 94, 613–619.
  12.  Jianjun Gao, Zhenxue Tian, Xu Yang.  Breakthrough: Chloroquine phosphate has shown apparent efficacy in treatment of COVID-19 associated pneumonia in clinical studies,  BioScience Trends. 2020; 14(1):72-73.