Andrographis paniculata, also known as green chiretta, belongs to the Acanthaceae family, has Ayurvedic origins, and is native to India and Sri Lanka. During the COVID-19 pandemic, China, Thailand, and other countries incorporated this herbal medicine into treatment guidelines and added it to essential medicine lists (Intharuksa et al., 2022). A. paniculata leaf extract has been used to treat ailments such as stomachache, inflammation, pyrexia, loss of appetite, irregular stools, and diarrhoea. The aerial parts have been documented in the treatment of the common cold, hypertension, malaria, and snakebites (Okhuarobo et al., 2014). A. paniculata has the potential for HDIs, particularly when taken in combination with drugs metabolized by CYP3A, CYP2C9, and CYP2C19, through both induction and inhibition (Sundhani et al., 2022). Balap et al. (2017) reported that co-administration of A. paniculata extract with naproxen, a CYP2C9 substrate in humans, decreased systemic exposure to naproxen in rat models. Systemic concentrations of drugs metabolized by CYP1A2 increased after co-administration with A. paniculata in rat models (Chien et al., 2010). With the increased use of this herbal medicine during the COVID-19 pandemic, HDIs with Paxlovid™ through CYP inhibition and induction, are possible.

Artemisia annua L., also known as sweet wormwood, is a member of the Asteraceae family and has a long history as treatment for various diseases. Extracts and isolated phytochemicals of A. annua have been reported to have anti-viral, pro-apoptotic, anti-inflammatory, anti-pyretic, antioxidant, and immunomodulatory activities (Soni et al., 2022). The sesquiterpenes produced by A. annua, artemisinin and artesunate, have shown high efficacy against multi-drug resistant malaria-causing parasites, resulting in artemisinin-based combination therapies for the treatment of P. falciparum malaria (Bosman et al., 2007). Artemisinin is known to be both an inducer and a substrate of CYP3A4 and CYP2B6 (Xing et al., 2012), leading to concerns about herb-drug interactions with artemisinin-based combination therapy. Based on its relatively broad range of biological activities, A. annua has also been used for the treatment of rheumatoid arthritis, systemic lupus erythematosus, allergic contact dermatitis, and has been evaluated for treatment of COVID-19 (Fuzimoto, 2021; Soni et al., 2022). A. annua extracts demonstrated inhibition of CYP3A4 activity (measured via 6ẞ-(OH)-testosterone) in Caco-2 cell monolayers, whereas artemisinin alone had no effect (Melillo de Magalhães et al., 2012). The same study also showed inhibition as well as a slight induction of CYP1A1 activity (measured via 7-ethoxyresorufin-O-deethylase) in Caco-2 cell monolayers (Melillo de Magalhães et al., 2012). A. annua tea infusions showed inhibition of CYP2B6 and CYP3A4 activity in HepaRG cells using P450-Glo assays (Kane et al., 2022). Based on the data available, HDIs with Paxlovid™ through CYP inhibition and induction with A. annua are possible.

Bowiea volubilis Harv. ex Hook.f. is a perennial, succulent plant commonly known as climbing onion, that belongs to the family Asparagaceae and is widely grown in dry regions of eastern and southern Africa (Ramarumo et al., 2019). Among the tribes of Southern Africa, the bulbous medicinal plant is traditionally used to treat various medical conditions, including dermatological disorders, sore eyes, urinary complications, infertility, and facilitation and induction of abortion (Steenkamp, 2003). Most preparations for therapeutic use involve mixing crushed fresh bulbs with water to form a decoction. A study by Fasinu et al. (2014) (Fasinu et al., 2014) showed that aqueous extracts of Bowiea volubilis inhibited the activity of CYP1A2 (phenacetin O-deethylation) and CYP3A4 (testosterone 6β-hydroxylation) in human liver microsomes, suggesting the potential for HDIs if these concentrations are attained in vivo. Studies have reported that all parts of B. volubilis contain alkaloids and several cardiac glycosides of the scillaren type (Mulholland et al., 2013). The mechanism of action of cardiac glycosides involves binding and inhibition of Na⁺/K⁺-ATPase with high selectivity and affinity. The CYP inhibitory activity of B. volubilis may be attributed to the alkaloids, which are substrates of human CYPs. A study by Salminen et al. (2011) showed that structurally related alkaloids from plants inhibited major human drug metabolizing CYPs (CYP3A4, CYP2D6, CYP2C19). Further research is needed to determine whether B. volubilis precipitates interactions with Paxlovid™ in patients and potential clinical consequences.

Turmeric is obtained from the rhizomes of C. longa L., a plant of the Zingiberaceae family that is native to tropical South Asia. The rhizomes are yellow-brown and are usually dried and ground to a yellow powder for use as a spice, herbal medicine, or dietary supplement. Ayurvedic medicine, Chinese medicine, and various pharmacopoeias describe uses of turmeric that include treatment of peptic ulcers, allergy, pain and inflammation, and liver disorders (Razavi et al., 2021). Turmeric is also under investigation for potential anti-SARS-CoV-2 activity and possible value in mitigating the COVID-19 cytokine storm (Rattis et al., 2021). The rhizomes of Curcuma longa contain monoterpenes and sesquiterpenes, including turmerone, arturmerone, and zingiberene, polysaccharides and curcuminoids. Turmeric dietary supplements usually contain extracts enriched in curcuminoids (up to 95% by weight) (Cheng et al., 2019). Some preclinical and clinical studies suggest that turmeric may produce pharmacokinetic HDIs. In a prospective study of 60 breast cancer patients, co-administration of turmeric with paclitaxel reduced serum AUC and C_max of paclitaxel by 7.7% and 12.1%, respectively (Kalluru et al., 2022). Although probably caused by induction of CYP3A4 activity, the effect was not considered clinically relevant. A study involving a human colorectal adenocarcinoma cell line showed curcumin inhibits P-gp, indicating the potential to alter the pharmacokinetics of drugs that are substrates for this efflux transporter (Flory et al., 2021). A clinical study involving eight healthy adult participants showed that curcumin increased the area under the curve (AUC) of sulfasalazine, a substrate for BCRP (Kusuhara et al., 2012). Several uridine 5′-diphospho-glucuronosyltransferase (UDP-glucuronosyltransferase, UGTs), including hepatic UGT1A1 and intestinal UGT1A8 and UGT1A10, metabolize curcumin to curcumin-O-glucuronide, which is a substrate for the basolaterally located efflux transporter MRP3 (ABCC3) (Jia et al., 2020). Therefore, pharmacokinetic turmeric-drug interactions may occur for drugs that are also substrates for MRP3 or these UGTs.

Glycyrrhizae radix et rhizoma (Chinese name, Gan cao), also known as licorice, is the dried root and rhizome of Glycyrrhiza uralensis Fisch., Glycyrrhizae inflata Bat, or Glycyrrhizae glabra L, and is a commonly used herbal medicine native to West Asia, North Africa, and Southern Europe (Wang et al., 2013; Liu et al., 2021). Licorice is used as an herbal medicine, dietary supplement, and food flavouring (Omar et al., 2012), and has been reported to exhibit various bioactivities, including anti-allergy, anti-inflammation, anti-virus, and anti-oxidant effects (Zhao et al., 2017). Studies have shown that licorice extracts or its bioactive phytochemicals can inhibit SARS-CoV-2 infection by affecting entry and replication of the virus, having potential for the treatment of COVID-19 (Sprouse and van Breemen, 2016; Yi et al., 2022). Various natural products and Chinese herbal formulae against COVID-19 are recommended by the Guidelines of Diagnosis and Treatment for COVID-19 issued by the National Health Commission of China. Licorice is one of the most frequently involved herbs in these herbal formulae or preparations against COVID-19, increasing risk for HDIs for anti-COVID-19 drugs (Lyu et al., 2021), and has been reported to modulate several CYP isoforms in vitro, including CYP3A4, CYP2C9, and CYP2E1 (Wang et al., 2013). Licorice extract was shown to activate human pregnane X receptor (PXR) (Haron et al., 2022) and induce CYP3A4 activity in human hepatoma cells (Wang et al., 2012). Regarding drug transporters, glycyrrhizin, one of the main bioactive ingredients of licorice, was shown to inhibit MRP2's uptake of S-(2,4-dinitrophenyl)-glutathione and induce P-gp. (Feng et al., 2015). Conversely, ritonavir is an inducer of uridine diphosphate (UDP)-glucuronyltransferase (UGT) and an inhibitor of P-gp and breast cancer resistance protein (BCRP), which may decrease efflux and in turn increase the metabolism of licorice phytochemicals (Marzolini et al., 2022). Given that licorice is one of the most used herbal medicines, in addition to a promoted treatment against COVID-19, there is potential concern regarding interactions with Paxlovid™.

Harpagophytum procumbens is a weedy, perennial, and herbaceous plant that is geographically distributed in the Kalahari Desert of Southern Africa. (Mncwangi et al., 2012). Harpogoside was isolated as the major phytochemical from H. procumbens tubers, amongst other iridoid glycosides. Both the pure isolated phytochemical (harpagoside) and extracts from the plant have shown potent anti-rheumatic, anti-inflammatory and analgesic effects (Georgiev et al., 2013). Products containing devil’s claw extract are widely used internationally by patients suffering from chronic low back pain and/or other chronic inflammatory diseases (Grant et al., 2007). Unger and Frank (2004) (Unger and Frank, 2004) conducted a study using baculovirus-infected insect cells to measure inhibition of the activity of six major CYPs by an H. procumbens extract, amongst other herbal extracts using LC/LC/MS. Inhibition of CYP1A2 and CYP2D6 by H. procumbens extract was comparably low, whereas CYP2C8, CYP2C9, CYP2C19 and CYP3A4 showed higher levels of inhibition. In another study, the effects of three commercially available devil’s claw products on P-gp activity were tested in a human proximal tubule (HK-2) cell line. (Romiti et al., 2009). Two of the products inhibited P-gp activity in calcein-AM tests and decreased protein expression via Western blot, albeit to a lower extent than the positive control, verapamil. These commercial devil’s claw products also inhibited esterase activity (via free calcein measurements), which may have resulted in an underestimation of their true P-gp inhibitory effects. However, harpagoside alone did not inhibit P-gp activity nor esterase activity (Romiti et al., 2009). Due to these in vitro interactions observed for devil’s claw through CYP and P-gp inhibition, Bordes et al. (2020) (Bordes et al., 2020) noted that H. procumbens may increase plasma concentrations of anti-retroviral drugs in patients, including those containing ritonavir, when administered concomitantly.

Hypoxis haemercollidea Fisch., C.A. Mey. (Hypoxidaceae), also known as African potato, is a tuberous perennial with star shaped yellow flowers that is indigenous to Southern Africa (South Africa, Lesotho, Eswatini, Zimbabwe, Botswana, and Mozambique) (Matyanga et al., 2020). Many species of Hypoxis are used as herbal medicines and are often morphologically similar (Ncube et al., 2013). Of these, H. haermercollidea is the best known and most studied. Decoction of H. haermercollidea corms is the most prominent herbal medicine preparation specifically used to treat HIV/AIDS as an immunomodulator (Mills et al., 2005a) and is also used to treat tuberculosis, cancer, headache, dizziness, ulcers, seizure disorders, depression, and anxiety (Ncube et al., 2013). Hypoxoside is the main phytochemical in African potato, which is readily converted to rooperol by β-glucosidase. Rooperol is the biologically active phytochemical that is attributed to the medicinal properties of the plant (Drewes and Khan, 2004; Mills et al., 2005a). Other notable biologically active phytochemicals are phytosterols and their glycosides, specifically β-sitosterol (Mills et al., 2005a; Nair et al., 2007; Matyanga et al., 2020). HDIs involving H. haemercollidea have been investigated, most focusing on antiretroviral drugs. African potato extracts mainly inhibited CYP3A4 (Mills et al., 2005b; Nair et al., 2007; Gwaza et al., 2009), as well as CYP1A2, 2A6, 2B6, 2C9, 3A5, and 2D6 in vitro assays (Nair et al., 2007; Gwaza et al., 2009; Fasinu et al., 2013). African potato extract showed dose-dependent activation of PXR (Mills et al., 2005b). For extracts high in hypoxoside, P-gp-mediated efflux increased compared to the ritonavir control (Nair et al., 2006). The P-gp-mediated efflux of nevirapine across human intestinal epithelial cells increased significantly in the presence of an African potato extract compared to vehicle (Brown et al., 2008), whereas the P-gp-mediated efflux of indinavir decreased substantially in the presence of H. haemercollidea (Havenga et al., 2018) compared to controls. Other studies showed no induction of P-gp at the concentrations tested (Gwaza et al., 2009; Fasinu et al., 2013). Co-administration of African potato with ART in human clinical trials did not alter the pharmacokinetics of efavirenz (Mogatle et al., 2008) nor lopinavir/ritonavir (Gwaza et al., 2013), which are substrates for and inhibitors of CYP2B6 and CYP3A4, respectively. Although the Gwaza et al. (2013) study showed no change in drug pharmacokinetics with concomitant use of Hypoxis obtusa, additional studies are needed to further elucidate potential concern with use of H. haemercollidea extracts co-administered with Paxlovid™, particularly regarding potential P-gp-mediated interactions.

Lessertia frutecens (L.) Goldblatt & J.C.Manning (previously Sutherlandia frutecens (L.)., R.Br) also known as cancer bush or Sutherlandia, is a flowering shrub that is a member of the Fabaceae family (Mills et al., 2005a; van Wyk and Albrecht, 2008). The plant is indigenous to South Africa, Lesotho, southern Namibia, and south-eastern Botswana and has been used by a wide range of cultures as a medicinal plant (van Wyk and Albrecht, 2008). Cancer bush has been used mainly to treat internal cancers—hence the name—as well as diabetes, inflammation, and infections, and to promote wound healing (van Wyk and Albrecht, 2008). More recently, Lessertia frutecens has gained popularity as antiviral treatment, specifically against HIV/AIDS as an immunomodulator (Mills et al., 2005a; Babb et al., 2007; Sibanda et al., 2016). Traditionally, decoctions or infusions are made from the leaves or bark (van Wyk and Albrecht, 2008). The main phytochemicals of L. frutescens are L-canavanine, D-pinitol, and gamma-aminobutyric acid (Mills et al., 2005a; van Wyk and Albrecht, 2008). Although research is limited, Lessertia has shown the potential to precipitate HDIs, specifically with antiretroviral drugs. Both water and ethanolic extracts showed inhibition of CYP3A4 (via conversion of dibenzylfluorescein to fluorescein) and activation of hPXR (via CYP3A4 luciferase reporter gene construct) and inhibited P-gp activity (via orthovanadate sensitive release of phosphate and adenosine triphosphatase activity) in HepG2 cells (Mills et al., 2005b). Similarly, Lessertia extracts inhibited P-gp in MDCK-MDR1 cells, reducing amprenavir efflux (Katerere, 2018), and modestly inhibited P-gp in Caco-2 cells using nevirapine as the substrate (Brown et al., 2008). In the same study by Brown et al. (2008), inhibition of P-gp by L-canavanine, a main phytochemical of L. frutescens, was observed. Extracts of L. frutecens inhibited a range of CYPs but mainly CYP3A4 and CYP3A5 (Fasinu et al., 2013). In LS-180 cells, CYP3A4 decreased after a short-term exposure to a Lessertia aqueous extract (less than 5 days); however, after 5 days of exposure, CYP3A4 activity increased 2-3 fold (Minocha et al., 2011). The lack of increase in CYP3A4 mRNA expression implied a post-transcriptional mechanism. In the same study, the pharmacokinetics of nevirapine were altered in rats. Specifically, the area under the curve (AUC) and maximum plasma concentration (C_max) decreased by 50% after 5 days of treatment with Lessertia (Minocha et al., 2011). Similarly, the AUC and C_max of atazanavir decreased in human participants (n = 12) after a single dose of Sutherlandia tablets (Müller et al., 2013). The activation of CYP3A4 and inhibition of P-gp by L. frutecens has the potential to elicit sub-therapeutic effects of the drugs administered, which may lead to either treatment failure or drug resistance. Given these potential interactions with CYPs and related pathways, there is evidence for potential HDIs with Lessertia.

Moringa oleifera, commonly known as moringa, is a tree that is native to India and widely cultivated throughout the tropics and subtropics because of its adaptability to different climatic conditions. (Kashyap et al., 2022; Liu et al., 2022). Previously reported analgesic, anti-inflammatory, antipyretic and immune boosting properties render moringa a potentially viable treatment for the management of COVID-19 (Chikowe et al., 2021; Nuertey et al., 2022; Siddiqui et al., 2022). However, some preclinical and clinical safety data indicate the potential for HDIs. Inhibition of CYP1A2 and CYP3A activity in pooled human liver microsomes has been reported. Using testosterone as the probe substrate, methanolic leaf extracts of M. oleifera inhibited CYP3A activity modestly, suggesting low interaction risk. Likewise, aqueous extracts showed weak CYP3A inhibition (Monera et al., 2008). Using fluorometric probes, methanolic extracts of M. oleifera leaves inhibited CYP3A and CYP1A2 activity (Awortwe et al., 2014), and aqueous extracts of M. oleifera leaves inhibited CYP1A2 and CYP2C9 activity. Inhibition of CYP3A and CYP1A2 activity was considered most likely because the IC₅₀s were higher than the minimum estimated dose warranting further investigation (Awortwe et al., 2014; Showande et al., 2019). Additionally, inhibitory effects of an ethanolic extract of moringa leaves on CYP1A2 and CYP3A4 activity were demonstrated in vitro (Taesotikul et al., 2010; Ahmmed et al., 2015). Two human clinical trials assessing the interaction between moringa and pharmaceutical drugs compared AUC and C_max of nevirapine with and without moringa coadministration, showing that moringa had no effect on the steady-state pharmacokinetics of the drug based on the FDA bioequivalence approach (USFDA, 2016; Monera-Penduka et al., 2017). In contrast, coadministration of moringa with amodiaquine resulted in a significant decrease (p = 0.037) in the time for amodiaquine to reach C_max. Several moringa phytochemicals have demonstrated antispasmodic activity, including glucomorigin, which may slow absorption, thus increasing time to reach C_max. In addition, other phytochemicals could form insoluble complexes affecting both C_max and the time to reach C_max (Olawoye et al., 2018). Although additional data are needed, some preclinical and clinical safety data indicate the potential for interactions between moringa and Paxlovid™.

Cat’s claw (Uncaria tomentosa) has shown beneficial effects against inflammation and viral infection, including COVID-19 (Steinberg, 1995; Erowele and Kalejaiye, 2009; Yuan et al., 2014; Yepes-Perez et al., 2021). Multiple in vitro studies have demonstrated cat’s claw as a CYP3A inhibitor (Budzinski et al., 2000; Sato et al., 2015; Weiss, 2018), but the specific phytochemicals have not been identified. A clinical study showed cat’s claw to inhibit CYP3A, as evidenced by increased plasma concentrations of antiretroviral drugs that are CYP3A substrates, including ritonavir (López Galera et al., 2008). In addition, a cell-based study showed that an extract of cat’s claw upregulated the protein expression of CYP2J2, CYP3A4, UGT1A3, UGT1A9, P-gp, and the apically located uptake transporter organic anion transporting polypeptide (OATP) 1B1 (SLCO1B1) (Weiss, 2018). Both nirmatrelvir and ritonavir are CYP3A substrates (Kumar et al., 1996; Li et al., 2011; Eng et al., 2022), suggesting risk for HDIs between cat’s claw and Paxlovid™ via CYP3A inhibition or induction.

Vernonia amygdalina is a small, perennial, rapidly regenerating shrub that grows wild in many sub-Saharan African countries, particularly Nigeria, Cameroon, South Africa, and Zimbabwe. Commonly known as bitter leaf, Vernonia amygdalina is widely consumed as a green leafy vegetable. An infusion of V. amygdalina leaves, sometimes with the young stems, is used to treat gastrointestinal tract conditions such as diarrhoea, constipation, and stomachache caused by helminthic, protozoal, and bacterial infection. Vernonia amygdalina is also used to manage malaria, diabetes, and cancer (Toyang and Verpoorte, 2013; Oyeyemi et al., 2018; Gyebi et al., 2021). Its use in managing fever and colds made it a candidate for screening as a potential source for a COVID-19 drug target (Gyebi et al., 2021). Vernonia amygdalina contains saponins, alkaloids, sesquiterpenes, and steroid glycosides, which are responsible for the plant’s biological activity (Farombi and Owoeye, 2011). Vernonia amygdalina has been shown to inhibit P-gp activity in vitro and to increase systemic exposure to the P-gp probe drug digoxin in vivo. Using Caco-2 cells, aqueous extracts of V. amygdalina leaves significantly inhibited P-gp-mediated efflux digoxin (p < 0.01) relative to control (Farombi and Owoeye, 2011; Oga et al., 2012). Using an isolated rat ileum model, V. amygdalina increased digoxin permeability in the mucosal-to-serosal direction by 43% compared to control (Oga et al., 2013). When V. amygdalina was co-administered with digoxin to rats, total plasma digoxin AUC was 2.1 times higher than that after digoxin was administered with vehicle. Caution may be warranted when V. amygdalina is co-consumed with other P-gp substrate drugs’ however, further research is needed to determine whether these pre-clinical results translate to the clinical setting.