Therapeutic Drug Monitoring in Inflammatory Bowel Disease

Abstract

Decades of cutting edge innovation in Inflammatory bowel disease (IBD) has yielded a diverse therapeutic armamentarium and warranted a shift in desired clinical endpoint (CE) from symptomatic management towards mucosal healing, histologic outcomes, serial biomarker trends and endoscopic remission. Despite these advancements, disease remission and therapeutic response rates remain suboptimal. This is due to failure to respond to therapy during the induction period (primary non-responder) or a subsequent loss of response (secondary non-responder). To address this area of unmet need, therapeutic drug monitoring (TDM) provides an opportunity to optimize dosing and therapeutic drug concentrations as per desired end clinical targets to improve response rates and offset aggressive disease complications. This further provides a platform for IBD therapeutic stratification based on patient, non-patient related factors and desired CE. In this chapter we aim to discuss a background regarding current TDM applications for various Food and Drug Administration (FDA)-approved IBD therapies and pinpoint deficiencies to enhance its smooth clinical implementation with a view to elucidating precision medicine as a novel therapeutic avenue in IBD.

1. Introduction

Inflammatory bowel disease (IBD) is a chronic remitting and relapsing disorder comprised of Crohn’s disease (CD), ulcerative colitis (UC) and indeterminate colitis [1]. Each disease subtype is classified based on location and extent of bowel wall involvement [1]. It is postulated that IBD-related intestinal inflammation is a consequence of aberrant immunological interaction between environmental, genetic and gut microbiome related factors [1]. In 1999, disease burden associated with IBD was estimated at 1.8 million (0.9%) in the American adult population [2]. However, as per recent extrapolations, the prevalence of IBD has substantially risen with as many as 3.1 million (1.3%) adults diagnosed with IBD [3]. As many as 70,000 new cases of IBD are diagnosed each year further exacerbating the disease burden [4]. Likewise, the prevalence of IBD has risen 2 to 3 folds in the US veteran population [5].

The swift incorporation of immunomodulators, biologics and small molecules have revolutionized the management of IBD. Current Food and Drug Administration (FDA) approved therapies include thiopurines, tumor necrosis factor (TNF) inhibitors (infliximab, adalimumab, certolizumab pegol and golimumab), anti-integrins (vedolizumab, natalizumab), anti-interleukins (ustekinumab, risankizumab), janus-kinase inhibitors (upadacitinib, tofacitinib) and sphingosine 1-phosphate receptor modulators (ozanimod) [6]. The clinical heterogeneity of IBD is attributed to complex immuno-pathophysiological responses [7]. Improved understanding of underlying complex mechanisms and the availability of disease modifying therapeutic agents have warranted a shift in desired clinical outcomes from symptomatic management to well defined end points such as mucosal healing, histologic healing, serial biomarker trends and endoscopic remission with the goal of achieving durable deep remission (Figure 1) [8].

Figure 1.

Defined clinical end points desired as per increasing management timeline with treat to target approach. Created by BioRender.com.

Despite a rapidly evolving therapeutic armamentarium, current literature suggests a perceived therapeutic ceiling attained with most therapeutic agents as evidenced by clinical remission rates leveling off at 40–50% [9]. Therapeutic response rates vary and remain suboptimal across drug classes. Only 50–60% of patients respond to therapy and obtain disease remission [10]. A further 50% of patients in remission eventually lose response to therapy [10]. This is attributable to absent clinical response post-induction (primary non-responder) or a loss of clinical response after initially attaining clinical response (secondary non-responder) [11]. Failure to therapy eventually results in the need for dose escalation or a switch in therapeutic class introducing higher odds of financial toxicity, recurrence of disease complications and hospitalizations [12]. To counter this unmet need, therapeutic drug monitoring (TDM) provides the opportunity to repurpose and optimize current modalities to directly improve patient outcomes.

TDM is a clinical decision making tool that quantifies serum drug concentration, active metabolites and anti-drug antibodies (ADA) to optimize treatment response [13]. Our current understanding of TDM is based on the results of multiple exposure-response studies that have delineated a positive association between higher drug/metabolite concentrations and desired clinical outcomes [13, 14, 15, 16]. TDM allows incorporation of person-to-person variability in pharmacokinetic (PK) and pharmacodynamic (PD) factors that contribute to a loss of response (LOR) [13]. LOR tends to occur in the setting of low drug concentrations with/without ADA or an overall failure despite adequate drug levels [13]. These respective readouts guide dosing to achieve therapeutic concentrations/drug exposure associated with maximum therapeutic benefit. Exposure-response data are available for almost all therapies. However, most exposure-response studies have analyzed only anti-TNF therapy [13]. More recently, studies assessing the utility of TDM with anti-integrin and anti-interleukin therapy have come forth [17, 18, 19, 20]. Innovation in antibody engineering have yielded newer biologics that are less immunogenic [21]. Therefore, the current rationale has also shifted from tackling immunogenicity to factors influencing drug exposure. Currently, almost all available IBD therapies have assays and TDM has become available for more modern therapies. Despite the implementation of TDM, there remains gaps that needs to be addressed to optimize its use in clinical practice. In this chapter we will review available evidence of TDM for various IBD therapeutic agents, utilization strategies and pinpoint deficiencies to improve current models of TDM [22].

2. Rationale for therapeutic drug monitoring

Our rationale for utilizing TDM is driven by data from multiple exposure-response studies suggesting strong association between clinical outcomes and serum drug concentrations [14]. Attaining optimal drug concentration is dependent upon several patient and drug related factors including but not limited to anti-drug antibodies, concomitant immunomodulator use, dose, disease phenotype, inflammatory burden, clinical end point, sample timing, sex, body mass and albumin levels [23, 24]. In a retrospective multicenter study, Juncadella et al. assessed biochemical, endoscopic and histological outcomes in 98 IBD patients treated with adalimumab who underwent TDM [25]. Findings from their study demonstrated that higher maintenance phase adalimumab serum concentrations >12 ug/mL were associated with both endoscopic and histologic remission [25]. Ungar et al. reported high rates of mucosal healing in patients with infliximab levels >6 ug/mL and adalimumab >8 ug/mL [26]. Similarly, Brandse et al. delineated a strong association between high serum drug concentrations in responders during induction with downstream desired clinical outcomes of endoscopic remission [27]. Likewise, studies have also showcased a poor association between low drug concentrations and clinical outcomes [14].

3. Mechanisms of drug failure

The availability of a myriad of therapeutic options can enforce premature cycling of drugs between different classes. This predisposes to premature drug discontinuation without effective dose optimization and incorrectly defining therapeutic failure. Particularly patients with refractory disease remain at risk of running short of therapeutic options with this approach. Therefore, it is imperative to better define and understand mechanisms underlying drug failure. This ensures maximum utilization of each drug class prior to a switch out of class.

3.1 Primary versus secondary non-response

Primary non-response (PNR) is defined as a lack of clinical improvement at the end of a defined induction period leading to discontinuation of therapy [28]. The incidence of PNR varies from 10 to 40% and is dependent upon the type of study design, definition of response and the type of therapeutic agent [29]. The time frames for assessment depends on the respective therapeutic class being utilized [28]. On the other hand, secondary non-response (SNR) is defined as worsening of symptoms indicating loss of response during the maintenance phase after having achieved clinical improvement during the induction period [28]. SNR is reported to occur in up to 50% of patients after 1 year of therapy [28].

The causative mechanisms for both are influenced by multiple factors including disease severity, disease phenotype, disease location, pharmacokinetics, pharmacodynamics, immunogenicity and dosing strategies [30]. Active smoking, low albumin, high inflammatory burden, presence of ADAs, high body mass index appears to lower response rates to TNFi (Table 1) [31]. Obesity also decreases time to LOR [32, 33]. It is postulated that obesity and low albumin increase drug clearance effectively reducing therapeutic drug concentrations [32, 33]. Patients with a history of prolonged disease duration (>2 years) are at a higher risk of developing PNR [34]. It is postulated that a longer disease duration is associated with a high burden of irreversible bowel damage impairing tissue integrity subsequently lowering response rates and patient outcomes [35]. Association between risk of PNR and disease location is well established in CD [36]. More specifically, small bowel and upper gastrointestinal CD is associated with a high risk of PNR [36]. Whereas colonic localization has shown improved response rates in patients undergoing TNFi therapy [36]. In summary, the underlying etiology remains poorly defined. However, the central theme remains inadequate drug concentrations during the induction or maintenance doses [28]. This is further backed by data from several recent studies delineating an association between inadequate serum drug concentrations and subsequent rates of therapeutic failure [14, 27, 37]. These risk factors need to be incorporated at the time of decision making to optimize risk based stratification for maximum drug utilization.

Factors	Effect
Weight	Increased drug clearance
Low Albumin	Increased drug clearance
>2 year disease history	Poor response rates due to effect on irreversible structural integrity and drug retention
Disease Type	Drug Clearance in UC > CD
Disease Activity	Drug clearance increased with higher inflammatory burden during active disease
Anti-Drug Antibodies	Increased Drug Clearance
Immunomodulator Use	Reduce Drug Clearance and formation of ADAs

Table 1.

Effect of risk factors on pharmacokinetics of anti-tumor necrosis factor therapy.

3.2 American gastroenterological consensus defined mechanisms

The AGA stratifies treatment failure based on drug trough and ADA levels in to three main causes including mechanistic failure, non-immune mediated pharmacokinetic failure and immune mediated pharmacokinetic failure [38]. As per the consensus determination, mechanistic failure also known as drug class failure should be suspected in patients not responding to therapy having attained optimal drug trough concentrations and undetectable ADA (Figure 2) [38]. These failures tend to present clinically as PNR early during the induction phase or later depicting a shift in underlying disease pathway as a mechanism to maintain ongoing inflammation [36]. This mechanistic escape eventually leads to the underlying disease process being driven by mediators not being targeted by that particular class of drug [38]. Therefore, the next best step in management for these patients represents a switch out of class to achieve response/remission [38].

Figure 2.

Framework to identify mechanisms of treatment failure in inflammatory bowel disease as proposed by the American Gastroenterological Association consensus. Created by BioRender.com.

The other 2 mechanisms pertain to pharmacokinetic (PK) failure and should be suspected in patients not responding to therapy in the setting of suboptimal drug concentrations [38]. They can be further sub-classified based on development of ADAs or non-immune drug clearance [38]. Non-immune mediated PK failure occurs in the setting of suboptimal drug concentrations and absent ADAs [38]. Non-immune failure can be mediated by sub-optimal dosing, low serum albumin, high body mass index, therapeutic non-compliance, high inflammatory burden associated drug clearance from reticuloendothelial system, gastrointestinal loss, drug wastage and varying drug distribution [14, 22, 38, 39, 40]. Patients presenting with non-immune failure can benefit from increasing drug dose or reducing dosing interval [41]. Immune mediated PK failure is suspected with high titers of ADAs and suboptimal drug concentrations [38]. The primary suspect for immune mediated failure are the drug neutralizing ADAs [38]. The most appropriate course of action in these cases is a switch within class [41].

4. Available tools for therapeutic drug monitoring

Knowledge of current commercially available assays are critical to the implementation of TDM in the clinical setting. Current models quantify drug concentrations by measuring the trough level (TL) and anti-drug antibodies (ADA). TL is considered as the point of lowest concentration in relation to the already established drug threshold cut-offs at which efficacy is expected (Figure 3). [42, 43]. Current available assays allow monitoring of most IBD biologics including all TNFi, vedolizumab and ustekinumab. The 3 most commonly used assays are the radioimmunoassay (RIA), enzyme linked immunosorbent assay (ELSA) and homogenous mobility shift assay (HMSA) [44]. The most economical and commonly used assay in clinical practice is ELISA [44]. Each assay can be further subclassified on the basis of differential pre-analytical steps, antibody capture and antibody detection [44].

Figure 3.

Representation of terminology with respect to desired concentration targets.

A natural question arises regarding the variation in performance of each assay. This led to multiple drug and anti-drug antibody comparative studies being conducted to assess individual performances [45, 46, 47]. Results from these studies helped establish the notion that good correlation exists among these assays for different drug concentration and anti-drug antibody titers [44, 45, 46]. Marini et al. evaluated infliximab concentration using four different commercially available ELISAs [48]. A strong correlation was noted with a coefficient of 0.89 for all tests [48]. When comparing inter-assay variability, ELISA, RIA, HMSA accurately detect serum drug concentrations (r = 0.91–0.97, p < 0.0001) [45]. Looking at individual assessment components, these assays are more reliable for producing specific, accurate and reproducible measures of drug TL [22]. The challenge remains in quantification of anti-drug antibodies due to variable readouts and requires careful derivation based on assay specific reference range [22]. Current guidelines recommend utilization of the same assay for checking both drug concentration and anti-drug antibodies in each patient [38].

5. Strategies for therapeutic drug monitoring

Prior to the availability of TDM, strategies involved in the management of therapeutic failure included empiric dose escalation, change within therapeutic class and switch outside therapeutic class. Even though this approach is thorough, it exposes the patients to trial with medications predisposing them to unnecessary adverse events. Additionally, these further delays initiating effective therapies and is associated with additional costs. While allowing for earlier implementation of effective therapies, TDM also provides a more personalized approach and is said to be more cost effective when compared to empiric therapy [49]. The utility of TDM varies based on clinical strategy. Current American Gastroenterological Association (AGA) guidelines recommend reactive TDM to guide dosing in patients with sub-optimal disease control [22]. Other possible clinical settings include proactive TDM during induction and maintenance phases, therapy de-escalation, drug holiday and toxicity prevention (Figure 4).

Figure 4.

Multiutility of TDM in inflammatory bowel disease. Created by Biorender.com.

5.1 Reactive therapeutic drug monitoring

Reactive TDM evaluates serum drug concentrations and ADA to identify the mechanism of primary or secondary LOR in the setting of active IBD or intolerance to therapy [50]. It is considered standard of care in patients losing response to therapy [13, 38]. The most common causes of therapeutic failure with TNFi therapy is non-immune PK failure (51%) followed by mechanistic failure (30%) and ADA-mediated PK failure (19%) [22] An algorithmic approach helps stratify patients on the basis of TL and ADA [50]. With undetectable drug levels and no ADAs, decreasing drug interval or increasing the dose can benefit the patient [22]. At the same time adding an immunomodulator to reduce immunogenicity and increased drug levels should also be considered. A mechanistic failure should be suspected when drug levels are adequate with or without ADA [22]. This signifies an alternative inflammatory pathway driving disease and warrants a switch to a different class of drug. Finally, undetectable drug levels with the presence of ADAs suggest ADA mediated failure [22]. In this scenario, our next step in management depends on the titers of ADA. If ADA titers are low, addition of an immunomodulator can help re-gain response. However, if ADA titers are high, switching within drug class or to another class altogether represent possible options. A schematic of the decision making tree is presented in Figure 5.

Figure 5.

Algorithmic approach to reactive TDM for biologic therapy.

In current literature, evidence regarding the utility of reactive TDM is stronger than that compared for proactive TDM. Additionally, most data pertains to the use of infliximab. Utilizing the algorithmic approach of reactive TDM helps better elucidate mechanisms associated with PNR. This provides an added benefit of minimizing inaccurate dose intensification in the setting of high ADAs [49]. In a retrospective study, Yanai et al. identified infliximab and adalimumab concentration cut offs at the time of SNR as an indicator of attaining downstream clinical benefit from a switch out of drug class or switching within class to another TNFi [40]. These cut offs are >3.8 ug/mL and > 4.5 ug/mL for infliximab and adalimumab respectively [40]. Roblin et al. utilized several infliximab decision algorithms as a model to extrapolate adalimumab pharmacokinetics [51]. At the time of SNR, adalimumab concentrations <4.9 ug/mL without ADA strongly predicted response to dose intensification in 67% of IBD patients [51]. Adalimumab concentrations >4.9 ug/mL were also associated with failure of 2 TNFi therapies [51]. Thus, adalimumab concentrations were also able to stratify patients that would require a switch out class [51].

In addition to rationalizing the treatment PNR and SNR, although controversial, we believe reactive TDM is non-inferior in terms of long term outcomes when compared with empiric therapy. In a retrospective observational study by Kelly et al., utilizing reactive TDM was associated with achieving a significantly higher post-adjustment clinical response and endoscopic remission [52]. Utilizing TDM was also found to lower rates of hospitalization [52]. In another study, Steenholdt et al. recruited 69 Danish CD patients receiving infliximab who had developed SNR [53]. They found similar rates of clinical response between patients with routine infliximab dose or TDM based intervention [53]. Interestingly, there was a 34% reduction in cost per patient in the TDM group [53]. The reduced cost of reactive TDM based dosing has also been well demonstrated in several other studies [54, 55].

5.2 Proactive therapeutic drug monitoring

Proactive TDM is defined as the assessment of drug trough concentrations and ADA at specific intervals to optimize dosing during induction, end of induction and maintenance phase [50]. The goal is to preemptively target optimal therapeutic range to prevent future LOR and disease flares [56]. This approach gained momentum due to data from exposure-outcome studies and post-hoc analysis of RCTs suggested an association between drug concentrations at specific time points and clinical outcomes when compared to reactive TDM and empiric dose escalation [25, 57, 58, 59]. The induction phase is a key period of interest due to high inflammatory burden, increased drug clearance and risk for inadequate drug exposure. Proactive TDM approach can help overcome these issues by early optimization of therapy [59].

In a study conducted by Papamichael et al., 101 UC patients were analyzed retrospectively [60]. The infliximab concentrations during induction was associated with short term mucosal healing [60]. In another multicenter study, outcomes were assessed for 102 IBD patients, patients undergoing proactive TDM had a greater drug persistence and reduced number of IBD related hospitalizations [61]. Papamichael et al. conducted another study in patients on adalimumab therapy where patients undergoing proactive TDM had a reduced risk of treatment failure compared to empiric therapy [22]. In the PAILOT study, a higher proportion of patients sustained steroid free remission and lowered biochemical markers with proactive adjustments of interval, dosing and drug trough concentrations when compared to reactive TDM [62]. In the landmark TAXIT trial, a lower frequency of undetectable drug levels, less antibody formation and lower disease flares were noted in the proactive TDM group [63]. The cost effectiveness of proactive TDM was recently assessed in a cost effective analysis of studies that utilized TDM with TNFi therapy [64]. Pooled assessment from 4 modeling studies, 1 prospective observational study, 1 retrospective observational study, 1 NRCT and 1 RCT favor cost-effectiveness of proactive TDM [64]. Finally, a framework to approach proactive monitoring is provided in Figure 6.

Figure 6.

Algorithmic approach to proactive TDM for biologic therapy.

5.3 Re-induction of therapy after drug holiday

A drug holiday is defined as a delay of ≥3 doses of biologic therapy [65]. Common reasons for a drug holiday include elective discontinuations due to surgery or infections, delayed insurance authorizations and stable remission [66]. In the setting of prior LOR to a biologic agent, re-induction with the same agent increases the risk of ADA formation and increases the odds of treatment failure. To prevent recurrence of treatment failure, the ACG endorses the use of proactive TDM to measure serum drug TL and ADAs after the first re-induction dose [67]. This also suggests that the greatest risk of infusion reactions is with the second re-induction dose [68]. In this scenario, TDM helps preempt PK failure due to possible inadequate drug exposure [67]. Currently, guidelines pertaining to the utility of proactive TDM for drug holidays are only established for infliximab [67].

Baert et al. evaluated 128 IBD patients who restarted infliximab therapy after a median discontinuation of 15 months [68]. Serum samples collected from prior trial of infliximab therapy were also analyzed for TL and ADAs [68]. Outcomes of this study included infusion reactions, TL, ADAs and response to treatment [68]. The absence of ADAs before second infusion and at re-initiation of therapy with immunomodulator use were associated with a clinical response [68]. Results from this study also suggested that early detection of antibodies to infliximab upon re-exposure resulted in higher infusion reactions [68]. Utilizing concomitant immunomodulator therapy to prevent development of ADAs and infusion reactions can help offset infusion reactions [68, 69].

5.4 De-escalate therapy

Commonly encountered in clinical practice, safety concerns warrant discontinuation of de-escalation from long term biologic and immunomodulatory combination therapy. In this setting, inappropriate de-escalation of biologic or immunomodulators can result in drop in biologic drug levels with a concomitant increase in immunogenicity. This increases risk of disease relapse and failure to maintain disease remission [69]. Proactive TDM can help measure both biologic and immunomodulator concentrations prior to treatment de-escalation. In the STORI trial, a low risk of relapse was seen in patients with low trough levels <2 mg/ul who discontinued infliximab after being on infliximab and thiopurine combination therapy [69]. Detectable infliximab trough levels at the time of immunomodulator discontinuation are also associated with long term response [70] Amiot et al. suggested de-escalation of infliximab therapy based on TDM rather than symptomatic and inflammatory burden stratification [71]. A real world cohort of 91 IBD patients in remission further showed that TDM is beneficial in delineating pharmacokinetic parameters helpful in the follow up of patients after biologic de-escalation [72]. As per the ACG consensus, dose de-escalation should be considered for infliximab or adalimumab for trough concentrations higher than 10–15 mg/ml [67].

6. Current guidelines for therapeutic drug monitoring

Despite the unavailability of large scale data, the use of TDM is supported by multiple medical societies globally [38, 56, 67]. In 2017, the AGA furnished guidelines to conditionally recommend the use of reactive TDM in adults with active IBD and receiving TNFi therapy [38]. These recommendations are limited to specific classes of TNF inhibitors and thiopurines. However, the use of proactive TDM is not currently recommended by guidelines [38]. Regardless, recent data suggests its active role in achieving favorable outcomes [61, 73]. Led by Cheifetz et al. the American College of Gastroenterology (ACG) published an expert consensus statement in 2021 [67]. In this report, the ACG encouraged the use of reactive TDM for all biologics [67]. They also recommended the use of proactive TDM for all TNFi therapy as well as under special circumstances such as drug holiday and therapy de-escalation [67]. Recommendations from ACG dug deeper in to utilizing TDM specific to disease subtype and activity [67]. In line with this, the ACG suggested the use of reactive TDM for UC relapse in patients with a history of moderate-severe UC [67]. However, no strong recommendations for CD were provided with the panel only suggesting reactive TDM in active CD [67].

7. Therapeutic drug monitoring for small drug molecules

7.1 Thiopurines

Initially developed for the treatment of acute lymphoblastic leukemia [74], thiopurines such azathioprine (AZA) and 6-mercaptopurine (6-MP) have proven to be quite effective in inducing steroid free remission in CD and UC [75, 76, 77]. In the current era, thiopurines are seldom used as monotherapy and are almost always used in combination with TNF inhibitors to enhance their therapeutic efficacy and minimize secondary LOR [78].

Thiopurines are pro-drugs that require extensive metabolism by multiple enzymes [79]. High inter-individual variability in its metabolism exists due to the requirement of specific target cell (leukocytes) and enzymatic activity [78]. AZA, 6-MP and thioguanines (TG) are converted into pharmacologically active 6-thioguanine nucleotides (6-TGN) namely 6-thioguanine monophosphate, 6-thioguanine diphosphate and 6-thioguanine triphosphate [78]. 6-TGN represents the pharmacologically active metabolites that eventually induce T-cell apoptosis [78]. Methylated metabolites are also of clinical interest as they are implicated in the development of thiopurine therapy associated toxicity [78]. Azathioprine undergoes direct conversion to 6-MP by removal of an imidazole group [78]. Subsequently, 6-MP gets metabolized to various metabolites via any one of three pathways [78]. The purine salvage pathways yields active 6-TGN metabolites by the enzyme hypoxanthine-guanine phosphoribosyl transferase [78]. Under the activity of thiopurine s-methyltransferase (TPMT), 6-methylmercaptopurine (6-MMP) are produced [78]. Incorporation of 6-TGN in to DNA and low TPMT activity have been associated with myelotoxicity [78]. High TPMT activity shunts the pathway towards increasing 6-MMP production, increasing drug toxicity and reducing downstream 6-TGN production resulting in reduced clinical efficacy [80]. Therefore, TPMT testing is important in determining if patients have reduced or absent TPMT activity. This ensures personalized dose reduction or selection alternative therapeutic agents. ACG recommends TPMT genotyping as it is a better representative of in vivo TPMT activity [67].

Dubinsky et al. proposed the utility of measuring metabolite concentrations (TGN and 6-MM) in erythrocytes along with pharmacogenomic profiling of TPMT genotype [81]. Despite the availability of tools to quantify these readouts, their role in the management of IBD are yet to be clarified [81]. A pooled analysis of 17 studies comprising of 2049 IBD patients established an association between 6-TGN metabolite levels and clinical remission [82]. A 3-fold increase in rate of clinical remission was noted in patients with 6-TGN levels >230–450 pmol/8 x 10⁸ erythrocytes [83]. A cut off concentration of 397 pmol/8 x 10⁸ erythrocytes have also been established for mucosal healing in CD [84]. Therapeutic non-compliance should be suspected if both 6-TGN and MMP are undetected [78]. Methodological and analytical assay limitations have limited further prospective assessments of thiopurine dose optimization in IBD [85, 86]. However, retrospective observational studies back the notion of introducing 6-TGN metabolite monitoring with clinical improvement noted in 6-TGN level driven dose optimization and significantly higher concentrations of 6-TGN in patients maintaining disease remission [87, 88]. Similarly, lower 6-TGN concentrations are also a feature of patients with active disease [88]. In addition to TPMT testing, thiopurine metabolite levels must also be quantified in a reactive setting and track toxicity [38]. As described above, cut off concentrations for thiopurine monotherapy have been well established. However, when used in combination with TNFi, 6-TGN cut off concentrations remain uncertain [38].

In the current therapeutic landscape, thiopurines are primarily used in conjunction with TNFi for sero-reversal of ADAs to reverse LOR, prevent LOR and immunogenicity to subsequent TNFi [89, 90, 91]. Their use has substantially declined due to the risk of malignancies [38]. More recently, studies have assessed the effect of proactive TDM on long term clinical outcomes. In a multicenter retrospective observational study, Barnes et al. assessed long term outcomes driven by thiopurine TDM interpreting 6-TGN and MMP metabolites [92]. In this largest retrospective cohort study of IBD patients receiving thiopurine, TDM was successful in optimizing dosage in 61.9% and enabled adherence to therapy [92] This approach of proactive TDM also improved clinical remission rates at the 1 year time point and accurately identified shunters that required addition of allopurinol to supplement therapeutic levels [92].

7.2 Methotrexate

Methotrexate is a folate antagonist (anti-metabolite) with antiproliferative, anti-inflammatory and immunosuppressive properties [93]. The efficacy of methotrexate monotherapy is well established in CD [94]. However, it failed to show any efficacy in UC [95]. The role of methotrexate is now limited to prevention of immunogenicity to TNFi. Adverse events associated with therapy include myelosuppression and hepatotoxicity [93]. A complete blood count and liver function test (LFT) must be ordered before and 1 month after initiation of therapy. Elevated LFTs warrant dose reduction whereas normal test results require maintenance of treatment with repeat testing every 3–4 months.

7.3 Sphingosine 1-phosphate receptor modulators

Ozanimod is a first in class sphingosine 1-phosphate (S1-p) receptor modulator that reduces circulating lymphocytes by blocking the egress from lymph nodes to inflamed tissue [96]. To date there are no exposure-response studies for this class of medication. The role of TDM remains to be further elucidated with data slowly emerging from efficacy and safety studies.

8. Therapeutic drug monitoring for biologics

8.1 Tumor necrosis factor inhibitors

Despite the availability of multiple agents in this drug class, our current understanding of the utility of TDM in TNFi is driven in majority by data from studies assessing infliximab or adalimumab. Very limited studies assessing exposure response relationships for certolizumab pegol and golimumab have been conducted to date. These studies have shown that higher drug concentrations are associated with better therapeutic outcomes in CD and UC respectively [97, 98, 99]. The role of TDM in infliximab have been thoroughly discussed in the strategies subsection.

With respect to proactive TDM in adalimumab, results from a multi-center retrospective study showed that the use of proactive monitoring performed superiorly than empiric dose escalation or reactive TDM by reducing rates of drug discontinuation, SNR, systemic adverse events or need for IBD related surgery [73]. In the PAILOT study, biologic naïve children who were responding to adalimumab therapy achieved higher rates of steroid-free clinical remission at week 72 when compared to reactive TDM [62]. From the perspective of reactive TDM, the EXTEND trial assessed the safety and efficacy of adalimumab in inducing and maintain mucosal healing [100]. It was concluded that patients undergoing reactive TDM were more likely to achieve mucosal healing [100].

8.2 Vedolizumab

Vedolizumab (VDZ) is a humanized IgG1 recombinant humanized monoclonal antibody approved for the treatment of moderate to severe CD and UC [101]. It prevents binding α4β7 integrins on naïve T and B lymphocytes and MADCAM-1 expressed on endothelial cells of the gastrointestinal tract and gut associated lymphoid tissue [101]. Hence, it tends to have a gut selective mode of action [101]. The therapeutic efficacy of VDZ was confirmed in the pivotal GEMINI I, II and III randomized control trials [102]. The recommended dose of VDZ is 300 mg with administrations at weeks 0, 2,6 during induction and every 8 weeks in the maintenance phase [102]. More recently, the efficacy and safety of subcutaneous administration of VDZ has been demonstrated in the VISIBLE trials [103]. Interestingly, the safety and efficacy profile of both standard IV and subcutaneous regimen are similar in both UC and CD for maintenance therapy [103].

When compared to TNFi, the correlation between clinical outcomes and VDZ concentration remain underpowered. During induction, initial association between VDZ concentrations and clinical efficacy were reported in the GEMINI trials [103]. Post-hoc analysis of GEMINI-I revealed a higher median CDZ trough levels during induction were associated with a significantly higher clinical remission rates at week 14 after induction therapy [104]. A weak association between week 6 VDZ trough concentrations and clinical outcomes with only a 22% remission rate in the higher quartile was noted in the GEMINI II and III trials [105]. To summarize, in UC patients, tough concentration increment from quartile 1 (≤ 17.1 ug/mL) to quartile 4 (> 35.7–140 ug/mL) was associated with a 31% increase in remission rate [106]. In CD patients, a 14% increase in remission rate was noted with a trough level increase from ≤16. ug/mL to 33.7 ug/mL [106, 107]. Studies have also helped demarcate VDZ trough cut-offs for mucosal healing and predicting the requirement for extended therapy. A trough level < 18.5 ug/mL was associated with a need for extended therapy in the first 6 months [108]. Whereas a trough level > 18 ug/mL was associated with mucosal healing [108]. As per a recent expert consensus, VDZ drug level > 33–37 ug/mL at week 6 is recommended during induction.

With respect to exposure-response relationship in the maintenance phase, patients in the highest VDZ concentration quartile experienced a 20% increase in clinical remission at week 52 when compared with patients in the lowest quartile [56]. No relationship was delineated in patients receiving VDZ on a q4 basis [56]. Taken together, the utility of VDZ TDM is still under investigation. Data suggests a possible exposure-response relationship. However, clear thresholds/cut-offs have not been determined. Based on these findings, target VDZ concentrations of 33–37 ug/mL (week 6), 15–20 ug/mL at week 14 (end of induction) and 10–15 ug/mL during maintenance phase have been recommended to achieve improved clinical outcomes [104]. To date no studies comparing reactive and proactive approaches have been conducted. Therefore, there remains a need for a high quality data to help establish the role of TDM in utilizing VDZ.

8.3 Ustekinumab

Ustekinumab (UST) is an anti-interleukin agent which binds to the p-40 subunit of both interleukin-12 and interleukin-23 [109]. This prevents downstream interaction between these interleukins and their IL-12RB1 receptor reducing IL-12 and Il-23 mediated cell signaling [109]. Similar to VDZ, although limited, exposure-response relationship studies have shown an association between higher UST concentrations and improved outcomes [110, 111, 112]. UST concentrations >4.2 ug/mL during induction (week 8) were associated with a > 50% decrease in fecal calprotectin [112]. UST concentrations during maintenance have also been shown to be associated with biomarker reduction and endoscopic response [101, 112]. A UST trough level > 4.5 ug/mL was associated with an endoscopic response and lower mean c-reactive protein levels at week 26 [101]. A post-hoc analysis of the UNITI I/UNITI II trials revealed an association between clinical remission and week 8 UST trough concentration (>3.3 ug/mL, AUC =0.57) [113]. Patients with UST trough concentrations in the two highest quartiles in the UNITI I/II trials reported higher rates of clinical remission [113].

9. Challenges in current therapeutic drug monitoring models

Several limitations restrict the widespread implementation of TDM in IBD management. Current proactive assessment is limited to studies involving infliximab and adalimumab. The majority of these studies are retrospective in nature and increases the influence of selection bias. There is insufficient evidence to establish a trough level cut off for other biologics such as VDZ, UST, golimumab and certolizumab. Despite the availability of multiple commercially available assays, lack of standardization and guidance for interpreting results represents a major hurdle. Furthermore, there are no established cut-offs for ADA to differentiate between high titer versus low titer. Drug trough levels must not be considered absolute as they can be easily influenced by several factors such as smoking, body weight, inflammatory burden, disease severity and phase of therapy. Lack of standardized guidelines can also complicate the transition of TDM into clinical practice.

10. Conclusions

TDM is a versatile platform with the potential to revolutionize and personalize management of IBD. It involves measuring serum drug concentrations and anti-drug antibody levels to stratify mechanism of therapeutic failure and optimize management approach in the setting of loss of response. Current guidelines and evidence supports the use of reactive TDM. However, further methodologically accurate powered studies are needed to ascertain the proactive approach.

Acknowledgments

There are no acknowledgements to declare. No funding was received for this scientific work.

Conflict of interest

The authors declare no conflict of interest.