Background/Purpose:

Patients with rheumatoid arthritis (RA) have an increased risk of developing osteoporosis. Dual Energy X-ray Absorptiometry (DXA) with assessment of BMD is used for diagnosis and monitoring of treatment effect. The access to DXA may be limited and changes in BMD occur only slowly, however. Therefore, the use of biochemical bone turnover markers (BTMs) to control compliance and treatment effect is gaining ground in daily practice. According to the International Osteoporosis Foundation (IOF) the most promising markers for clinical use are s-P1NP (serum procollagen type 1 N-terminal propeptid, µg/L) and s-CTX (serum procollagen type 1 N-terminal propeptid, µg/L) reflecting the bone formation and resorption, respectively. When treating with bisphosphonates, the BTMs are expected to be suppressed already after a few months. However, analytic and biologic variation may influence the results. TNF-inhibitors reduce bone loss and reduce serum levels of P1NP and CTX. The use of BTMs in RA patients treated with anti-osteoporotic agents is tempting, but there is no data on the reliability of BTM during TNF-inhibitor treatment cycles.

Objective:

To examine the reliability of CTX and P1NP in a group of stable RA patients during a cycle of TNF inhibitor treatment.

Methods:

27 RA patients with receiving TNF-inhibitor treatment were identified in the Danish Registry for biological treatment (DANBIO). None of the patients were treated with anti-osteoporotic agents. Measurements of fasting morning s-P1NP and s-CTX were performed 1-2 days before and after administration of the biological agent and in the middle of the treatment cycle. Analyses were performed according to standard procedures. Between-test differences were examined using Student’s t-test for paired data. Reliability was expressed as the coefficient of variation (CV) % calculated as = 100 x √(∑s_i²/(n-1))/mean and as the corresponding minimal detectable difference (MDD) % calculated as = 1.96 x √2 x CV%). The MDD is the difference between two measurements which would be statistically significant when applied to a reference group (level of significance 0.05).

Results:

Age was 61.6±12.6 years, DAS28 2.8±0.99 (mean±1SD). 57% of the patients were treated with adalimumab, 18% with certolizumab, 14% with etanercept, 11% with infliximab and 70% with a DMARD (methotrexate (n=17), azathioprine (n=2)). Results of P1NP and CTX measurements are shown in the Table.

The CV for P1NP was 18.7 % and the corresponding MDD 52.0 %. The CV for CTX was 20.3% and the MDD 56.3 %.

Conclusion:

The biochemical bone markers P1NP and CTX were stable on the group level during cycles of TNF-inhibitor treatment in patients with RA. CVs and MDDs were high, however. If treatment with anti-osteoporotic agents is initiated, changes in both P1NP and CTX below 50% in the individual patient may be due solely to measurement error and should therefore be interpreted with caution.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/reliability-of-the-biochemical-bone-markers-ctx-and-p1np-during-a-tnf-inhibitor-treatment-cycle-in-patients-with-rheumatoid-arthritis/