Single Nucleotide Polymorphisms Linked to P-Glycoprotein Function
Several single nucleotide polymorphisms SNPs have been linked directly to active transporters, such as
rs3842. It has been associated with improved survival rates in chronic myeloid leukemia patients treated with dasatinib [
21], but not plasma concentrations of dilutegravir in HIV-positive patients [
22]. Though there is some evidence that the gene is associated with efavirenz plasma concentration in Tanzanian and Ethiopian patients [
23], there was no association observed in a Brazilian study of predominantly African ancestry [
24]. Zaorska et al., (2021)[
25] further established a link between
rs1922240 and nephrotic syndrome, where the rare G allele is associated with the occurrence of childhood nephrotic syndrome, steroid dependence (for the AAC haplotype) and mesangial proliferative glomerulonephritis (wild A and AA alleles).
Similar impacts on plasma concentration and toxicity have been seen with
rs1045642 although there are conflicting results. Higher methadone levels were highlighted as a potential risk factor during opioid replacement therapy by Iwersen-Bergmann et al. (2021)[
26], linked to higher pain sensitivity by Zahari et al. (2017)[
27] in Malay and as a protective factor for depression in a Chinese sample (Xie et al. 2015)[
28]. This contrasts with a Japanese sample which demonstrated increased depression rates in carriers [
29]. Though Menu et al. (2010)[
30] found no impact on antidepressant therapy efficacy or tolerance, others found that a specific polymorphism of
rs1045642-TT predicted lower effective doses of escitalopram and increased response from venlafaxine [
31,
32]. However, Kato et al. (2008)[
33] suggested that carriers might have poorer responses to paroxetine, suggesting differential treatment responses based on genetic factors. Carriers are associated with higher clozapine serum concentrations with an associated body mass index increase [
34], less fatigue and fewer sleep disorders in prolactin adenoma patients treated with cabergoline [
35], higher rates of seizures but lower incidence of posterior reversible encephalopathy syndrome (PRES) in children with lymphatic leukemia undergoing treatment [
36] and neuropsychiatric adverse reactions in oseltamivir-treated children with influenza H1N1/09 [
37].
Extensive investigations have been carried out on the
rs2032582 allele. Studies on a humanised mouse model with the 2677G>T mutation revealed no change in P-glycoprotein expression levels in brain capillary fractions, yet this mutation resulted in increased brain penetration of verapamil, a representative substrate of P-glycoprotein [
38]. Moreover, placentas from mothers with the TT/TT genotype exhibited significantly reduced P-glycoprotein expression [
39]. A correlation was also observed between the binding potential of verapamil and the dosage of T alleles [
40], though two studies have found no link to donepezil efficacy in Swiss and Italian Alzheimer’s disease patients [
41,
42]. The allele has been associated with a higher likelihood of requiring increased methadone doses [
43] and improved methadone plasma concentrations and treatment outcomes [
44] in Taiwanese patients, thus influencing both the kinetics of methadone-P-glycoprotein interaction and methadone’s potency [
45,
46]. The need for higher doses of methadone was also reported in Han Chinese [
47]. These data suggest the
rs2032582 allele increases methadone potency via increasing BBB permeability, which is of considerable clinical significance given the high inter-individual variability in methadone response coupled with the significant toxicity of methadone in terms of cardiac and CNS effects, as well as potential drug-drug interactions via being a substrate for various CYP450 enzymes relevant to psychotropics (i.e. 2B6, 3A4, 2D6) [
48].For antidepressants, the polymorphism significantly affects both citalopram plasma and cerebrospinal fluid concentrations [
49]. The results are more conflicting for paroxetine, where it was associated with paroxetine response in a Japanese cohort [
33] but not Swiss [
50] or Slovakian cohorts [
51].
In studies of antipsychotic medications and
rs2032582, Geers et al. (2020)[
52] found a protective effect against antipsychotic-induced hyperprolactinemia in a subgroup of schizophrenia patients treated with risperidone or paliperidone. Cho et al. (2010)[
53] reported significantly higher pharmacokinetic parameters following levosulpiride administration, increased sleep duration was observed in Russian adolescents during episodes of psychosis [
54], and Kuzman et al. (2008)[
55] reported that women with schizophrenia were less prone to significant weight gain during risperidone treatment. Though Skogh et al. (2011)[
56] demonstrated elevated serum and CSF olanzapine concentrations, Consoli et al. (2009)[
57] noted lower clozapine concentrations, and Rafaniello et al. (2018)[
58] found a lower aripiprazole concentration-to-dose ratio. Additional research has been carried out by Suzuki et al. (2014)[
59];
CYP2D6-variant carriers had increased aripiprazole concentration and dose-ratios compared to non-carriers. These findings were corroborated by Belmonte et al. (2018)[
60], Hattori et al. (2018)[
61] and Koller et al. (2018) who identified that the variants
rs1045642, rs2235048, rs1128503, rs2032582 as especially relevant[
62]. on aripiprazole and by Xing et al. (2006)[
63] and Yasui-Furukori et al. (2007)[
64] on risperidone.
Zimprich et al. (2004)[
65] reported a significant link between drug resistance and the
rs2032582 allele in epilepsy patients, later confirmed by Hung et al. (2005)[
66]. Though Vahab et al. (2009)[
67] and Sun et al. (2016)[
68] found no significant association between these genetic polymorphisms and antiepileptic drug resistance, Lovric et al. (2012)[
69] observed higher lamotrigine concentrations in Croatian epilepsy patients with this polymorphism and Smolarz et al. (2017)[
70] identified a significant association with drug-resistant epilepsy in the Polish population. Others separately reported associations between genetic variations and drug-resistant epilepsy in Mexican, Tunisian, Han Chinese, and Uygur populations, respectively [
71,
72,
73,
74].
Other indirect associations between BBB and drug response are observed. For example, there is a further association with increased drug response and decreased mortality in multiple myeloma patients [
75] in
rs2235013 carriers, increased cyclosporine levels in the months following heart transplant [
76] and chemotherapy response and recurrence-free survival in late-stage lung cancer patients [
77]. A similar treatment response in chemotherapy-treated stage III lung cancer has been seen for
rs2235046, which is also associated with non-inflammatory Crohn’s disease in children [
78] and ulcerative colitis in adults [
79]. Finally,
rs2235033 was not found to be associated with methotrexate response in halo-hematopoietic stem cell transplantation in paediatric patients with malignant haematological diseases [
80].
SNPs Linked to Drug Transport
Examining the alleles associated with wider drug transport, Margier et al. (2019)[
81] observed an association between
rs868755 and increased plasma concentration of 25-hydroxycholecalciferol. Though there has been some evidence of an association between
rs868755 carriers and methotrexate survival and toxicity in rheumatoid arthritis patients, this has been shown to be dependent on specific allele variants [
82], and no association was found with drug resistance or sensitivity among children with refractory epilepsy [
83]. Meanwhile, the
rs1202168 variant is potentially related to the development of abnormal haline dispositions in mesial temporal lobe epilepsy [
84] and increased risk of colorectal cancers in two predominantly Caucasian samples (German and Czech; [
85]), but might be protective against colorectal cancers in German menopausal hormone replacement therapy patients [
86].
Sági et al. (2021)[
36] found an association between
rs1128503 and higher rates of seizures and CNS relapse in children with lymphatic leukemia, particularly with the CC allele, which trends towards worse outcomes at six months compared to other variants, as reported by Cousar et al. (2013)[
87]. Though investigated in several other settings, it has not been linked to methadone concentration [
26], severe traumatic brain injury (TBI) recovery [
87], escitalopram, venlafaxine [
32] or paroxetine [
33] efficacy. The ABCB1 genotypes of
rs1922242 exhibited a significant association with the severity of depressive symptoms in major depressive disorder (MDD) patients who underwent continuous escitalopram treatment for 8 weeks [
31]. More concerning is that women homozygous for the
rs2214102 allele were found to be at a significantly increased breast cancer risk when taking combined estrogen-progestogen contraceptive pills [
88]. Likewise, patients with breast cancer carrying the ancestral homozygous genotype (GG) had significantly worse progression-free survival than carriers of the non-ancestral allele [
89].
A series of alleles, namely
rs1211152, rs2214103, rs2235018, rs2235020 and
rs2235074, have been investigated for tramadol response and some responses have been indicated, though no evidence has been provided to date. At least two genetic studies failed to detect or observe
rs1211152 in their samples; one from Africa comprising Xhosa, Afrikaans, and English ancestry [
90], and another from the United States [
91]. Evidence from Dong et al. 2011 suggests an minor allele frequency of <2% among Mexican-American samples, whereas McMahon et al. (2010)[
92] observed an MAF of around 9% in a German sample of European ancestry. By comparison,
rs2235020 has been associated with improved rates of remission of MDD in a sample of Mexican Americans treated with fluoxetine [
93], and
rs2235074 is potentially related by haploblock to xenobiotic efflux in myeloma [
94].
The
rs2235035 allele has been observed as associated with increased daunorubicinol clearance in a paediatric oncology sample [
95] and with atorvastatin pharmacokinetics and lipid-lowering response in a healthy Korean sample [
96]. While
rs3213619 has been associated with grade two neurotoxicity [
97], it was not associated with survival or prognosis in adriamycin cancer patients treated with taxane-containing regimens [
98]. The variant is also potentially protective against paclitaxel-induced peripheral neuropathy [
99], particularly in breast cancer [
100].
Louis et al. (2022)[
101] have reported an associated decreased seizure occurrence after surgical resections for drug-resistant temporal epilepsy in people with rs10276036. Furthermore, Caronia et al. (2011)[
102] found a strong association between the genotype and increased five-year survival rates in osteosarcoma patients treated with a combination of cisplatin, adriamycin, methotrexate, vincristine, and cyclophosphamide, suggesting that the genetic marker might have some prognostic value in cancer treatment. Similarly, Weissfeld et al. (2014)[
77] reported an association between the genotype and increased survival and disease-free status at four years post-diagnosis in stage III-IV lung cancer patients treated with chemotherapy, suggesting potential implications for treatment outcomes and prognosis in lung cancer patients. However, genotype TT is strongly associated with an increased risk of nevirapine drug hypersensitivity in people with HIV infections [
103,
104].
Despite extensive research, most studies have not demonstrated an association between
rs2032588 and psychotropic response or side effect profile. A large (n=789) naturalistic Dutch cohort study demonstrated a reduction of PGP-dependent antidepressant (citalopram, venlafaxine, paroxetine, fluvoxamine) side effects [
105]. Additionally, a Jordanian cross-sectional study found an association between
rs2032588 and anticonvulsant drug resistance in a cohort of 86 Jordanian men, but not women, with epilepsy [
106].
Rs2032588 is much more prevalent in African populations (~18%) compared with European (~6%), and virtually absent in Asian populations [
107].
SNPs Linked to Drug Response and Pharmacokinetics
Limited research appears to have been conducted for the SNPs, such as
rs2888599, associated with drug response except in tramadol as carried out by the Bruce Budowle Laboratory (University of North Texas Health Science Center;[
108]), though results do not appear to have been published yet. The presence of
rs4148727 was not associated with an increased risk of ischaemic stroke in a South Korean sample but was associated with reduced severity [
109]. Some evidence suggests that the SNP is associated with the lipid profile in type II diabetes mellitus, most strongly for apolipoprotein-A and triglycerides, with likely implications for statin management in the condition [
110]. No significant association was observed for an improvement in clopidogrel efficacy compared to aspirin for stroke prevention in a large Chinese sample of high-risk patients [
111]. Unfortunately,
rs13237132 has no available evidence directly addressing drug response in this SNP, though despite some earlier findings, it does not appear to be associated with ovarian cancer outcomes [
112], particularly when treated with taxane [
113].
This is similar to
rs9282564, which has had several investigations, though little corroborating findings from the clinical literature. Indirect evidence suggests that the SNP might be protective against opioid overdose ([
114]). However, children undergoing tonsillectomy using intravenous morphine with GG and GA genotypes of ABCB1 polymorphism
rs9282564 had higher risks of opioid-related respiratory depression, [
115]). Additionally, Ray et al. (2015)[
116] found an association between the SNP and a greater likelihood and faster remission of MDD with sertraline treatment. However, Sánchez-Lázaro et al. (2015)[
117] reported an association between the SNP and significantly lower renal function after heart transplantation when treated with calcineurin inhibitors (tacrolimus or cyclosporine), which could be a potential risk factor for adverse outcomes in transplant patients. Burgueño-Rodríguez et al. (2023)[
118] identified an association between the SNP and increased risk of neurotoxicity in paediatric acute lymphoblastic leukemia patients treated with prednisone, vincristine, L-asparaginase, daunorubicin, and methotrexate, indicating potential limitations or risks associated with this treatment regimen.