Introduction
The severe acute respiratory syndrome coronavirus (SARS-CoV) are single-stranded positive-sense RNA viruses that cause severe respiratory diseases to the affected individuals (Cheng et al. 2007). In December 2019, a cluster of pneumonia cases emerged in Wuhan, China (Huang et al. 2020). This disease is now known as coronavirus disease 2019 (COVID-19) caused by the novel coronavirus now known as SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) that has spread globally, affecting a large portion of the human population across the world (Cohen and Normile 2020; Zhu et al. 2020).
The full range of symptoms for COVID-19 includes self-limiting respiratory tract illness to severe pneumonia, acute respiratory distress syndrome (ARDS), multi-organ failure, and death (Huang et al. 2020; Ruan et al. 2020a; Wang et al. 2020a). However, with time as the number of COVID-19 positive patients increase across the world, few neurological symptoms such as headache, paresthesia, and consciousness disorders were reported as well (Wu et al. 2020b). More recently, unusual manifestations of COVID-19, including encephalitis, (Ye et al. 2020), acute necrotizing hemorrhagic encephalopathy (Poyiadji et al. 2020), and myocarditis (Doyen et al. 2020) have been documented. Besides, it has been noted that thrombotic complications in COVID-19 ICU patients increased remarkably (Klok et al. 2020). Lastly, skin manifestations like erythematous rash, urticaria, or chickenpox-like vesicles mainly in the body trunk in COVID-19 patients were reported in multiple studies (Joob and Wiwanitkit 2020; Recalcati 2020).
Therapeutic options to contain the COVID-19 pandemic is urgently needed. Favipiravir (T-705) (Wang et al. 2020b) and ribavirin have been evaluated on COVID-19 patients (ChiCTR2000029387 ), but ribavirin reported side effects (Zumla et al. 2016). Remdesivir (GS-5734) has been suggested (Al-Tawfiq et al. 2020; Cao et al. 2020b) and a compassionate-use remdesivir study showed 68% clinical improvement in COVID-19 patients (Grein et al. 2020). However, very recently, WHO reported controversy to the aforementioned data, and a full COVID-19 clinical trial of remdesivir was terminated due to the adverse side effects (unpublished report from WHO website). In addition, lopinavir–ritonavir treatment on COVID-19 patients did not show any improvement (Cao et al. 2020a). However, chloroquine, hydroxychloroquine (Gao et al. 2020; Gautret et al. 2020) and azithromycin with hydroxychloroquine showed potential clinical benefits but only in a limited number of COVID-19 patients (Gautret et al. 2020). Tocilizumab (Xu et al. 2020), as well as convalescent plasma therapy (Duan et al. 2020) in severely ill COVID-19 patients, also improved clinical outcomes, but inadequate clinical data to justify the observed effect. Although a range of the aforementioned therapies can be a near-term strategy to tackle COVID-19, there is still an evident lack of specific treatment for COVID-19 (Huang et al. 2020).
Methylxanthines are heterocyclic compounds that are methylated derivatives of xanthine comprising of coupled pyrimidinedione and imidazole rings (Talik et al. 2012). Methylxanthines have been widely used for therapeutic purposes for decades, with proven therapeutic benefits in different medical scopes. For example, the naturally occurring methylxanthines like caffeine, theophylline, and theobromine have been used in the treatment of respiratory diseases (Lam and Newhouse 1990), cardiovascular diseases (Batterman et al. 1959), cancer (HAYASHI et al. 2005; Kimura et al. 2009) and the commercially produced xanthine derivative drug like pentoxifylline has been widely documented to have immunomodulatory properties including the downregulation of Tumour Necrosis Factor (TNF) α to treat the injurious effects due to immune activation in the syndrome of chronic heart failure (CHF) (Shaw et al. 2009).
Pentoxifylline and its active metabolites enhance blood flow by decreasing blood viscosity and ameliorating erythrocyte flexibility. Administration of pentoxifylline produced hemorheological activity in a dose-dependent manner. Based on the aforementioned mode of action, pentoxifylline has been approved to treat intermittent claudication due to chronic occlusive arterial disease of the limbs (Dettelbach and Aviado 1985). In these patients, pentoxifylline improves microcirculation and tissue oxygenation (Hsu et al. 1988; Harris et al. 2017). Pentoxifylline is also used for the management of alcoholic hepatitis (severe) (Whitfield et al. 2009) and venous leg ulcer off-label (Coccheri and Bignamini 2016; Zito and Murgia III 2018). Moreover, the effect of pentoxifylline has been demonstrated to treat fibrotic lesions by immunomodulation and by reducing inflammation (Wen et al. 2017).
Previous research has extensively established the effects of caffeine in the treatment of respiratory disease, its bronchodilatory effect via phosphodiesterase (PDE) inhibition, and adenosine receptor antagonism (Sullivan et al. 1994; Tilley 2011). Furthermore, caffeine is widely used to treat apnea of prematurity (AOP) in preterm infants by improving minute ventilation, CO2 sensitivity, respiratory muscle function, and neural respiratory drive. Caffeine administration also improved microcirculation in humans (Okuno et al. 2002), and moderate caffeine consumption was related to reduced coronary heart disease and stroke (Bøhn et al. 2012). Therapeutic indications of caffeine also include its role as the CNS stimulant to maintain seizure control during epilepsy (van Koert et al. 2018) as well as its role in treating headaches. As an adjuvant to analgesics, it enhances the efficacy of analgesics to treat headache (Lipton et al. 2017). Besides, the OTC labeling of caffeine is to restore mental alertness or wakefulness during fatigue (Childs and de Wit 2008). Despite the numerous benefits of caffeine, high doses of caffeine may lead to anxiety disorder (Lara 2010), and patients with an anxiety disorder are more sensitive to caffeine (Bruce et al. 1992).
Herein, we review with extensive evidence that widely used methylxanthines like caffeine and pentoxifylline may be used as an adjuvant therapy to treat COVID-19 induced respiratory symptoms by exploiting their reported immunomodulatory and anti-inflammatory potentials. Tissue oxygen levels have also been shown to be significantly increased by therapeutic doses of pentoxifylline in patients with peripheral arterial disease.